Puberty is started by certain genes and hormones in the body.

It’s not yet fully understood why some people experience puberty earlier or later than others, although there are a number of possible factors.

Genes

Research has found that two genes present at birth, known as GPR54 and KiSS1, are responsible for the onset of puberty.

The GPR54 lies dormant (inactive) for many years until it’s suddenly activated by special chemicals called kisspeptins that are produced by the KiSS1 gene.

The process of puberty starts when kisspeptins turn on the GPR54 gene, sending signals to your brain and triggering a chain reaction in your body.

An area of the brain known as the hypothalamus activates a hormone called gonadotropin-releasing hormone (GnRH). GnRH sends a signal to the pituitary gland (a pea-sized gland near the base of the brain) to release hormones that stimulate the ovaries (in girls) or testicles (in boys) to make sex hormones.

This chain reaction and release of hormones brings on the changes of puberty.

Hormones

The ovaries and testicles produce two sex hormones  responsible for changes that occur during puberty.

In boys, testosterone is produced by the testes (male sex organs). Testosterone stimulates the development of the penis and testes and causes muscle and pubic hair growth. It’s also responsible for lowering the voice.

Women and girls also have testosterone in their body, which is produced in small amounts by the ovaries to help maintain muscle and bone strength.

Oestrodiol is the main sex hormone in girls. It’s produced by the ovaries and stimulates growth of the breasts and reproductive system, and helps regulate the monthly menstrual cycle (periods).

Boys and men also have oestrodiol in their body. It’s produced in small amounts by the brain and testes to help maintain bone density.

Triggers of puberty

It’s thought puberty may be triggered by environmental and genetic factors.

Studies have shown that on average, black girls start puberty earlier than white girls. But there’s no evidence to show black boys mature faster than white boys.

Diet and nutrition are also thought to be important factors, particularly in girls. Studies have shown that girls who are overweight or obese tend to start puberty earlier, while girls with a lower body weight tend to start later.

The rising trend of obesity in girls could explain why the average age of girls beginning puberty has been falling over recent years. But it’s not known why obesity doesn’t have the same effect in boys.

There’s a lot of uncertainty regarding why certain factors seem to trigger puberty and research in this area is ongoing.

Read more about early or delayed puberty.

Some children experience puberty earlier or later than others.

There are several possible reasons for this. In some cases, early or late puberty could be a sign of an underlying condition and tests may be needed.

Early (precocious) puberty

An unusually early puberty is where the symptoms of puberty, such as breast development, enlargement of the testes and pubic hair growth, start before eight years of age in girls and nine years of age in boys.

The start of puberty is usually triggered by the GPR54 gene, which sends signals to your brain and starts a chain reaction and release of hormones in your body. The early start of this chain reaction can be caused by:

• a problem in the brain, such as a tumour
• a brain injury due to head trauma
• a brain infection, such as meningitis
• a problem in the ovaries or thyroid gland
• an inherited tendency (it may run in your family)

But for most girls there’s no known reason for starting puberty early. In boys, early puberty is less common and more likely to be associated with an underlying medical problem.

Treatment for early puberty

To diagnose the cause of early puberty, your GP may recommend that you have a blood test to check for any problems with your hormones.

Your GP may refer you to a paediatric specialist for further assessment and tests. These may include an ultrasound scan and magnetic resonance imaging (MRI) scan to check for tumours and the correct functioning of glands and organs.

Depending on the cause, early puberty can be treated by:

• treating the underlying cause
• using medication to lower the high levels of sex hormones and stop sexual development progressing

Treatment with medication is only usually recommended if it’s thought that an early puberty would cause significant current problems for the young girl. However, this usually isn’t the case, unless puberty started well before eight years of age.

If puberty started particularly early (before six years of age), it will usually only result in the girl being short. In most cases, an early puberty won’t usually cause any significant health problems.

However, early puberty can cause behavioural issues which can have a disruptive impact on the child’s education. You may want to discuss this further with your doctor. This may be particularly relevant for girls with additional special educational needs (SEN).

Delayed puberty

In girls, an unusually late puberty is diagnosed if:

• there are no signs of breast development by 13 years of age
• four years have passed since the start of puberty and the breasts haven’t reached full adult development
• a girl hasn’t had her first period by 14½ years of age

In boys, an unusually late puberty is diagnosed if:

• there are no signs of testicular development by 14 years of age
• 3-4 years have passed since the beginning of puberty and the penis and testicles haven’t reached full adult development

The delayed onset of puberty can be caused by:

• an inherited tendency (late puberty may run in your family)
• having a long-term illness, such as cystic fibrosis, diabetes, or kidney disease
• malnutrition, possibly from an eating disorder, or a chronic illness such as cystic fibrosis
• over-exercising, such as in the case of professional athletes and gymnasts
• tumours or other internal damage to your glands
• hormonal conditions, such as an underactive thyroid gland
• a genetic condition that affects sexual development, such as androgen insensitivity syndrome (a rare condition where a person is genetically male, but their body is insensitive to male sex hormones)

Treatment for late puberty

If there’s no obvious cause for delayed puberty, such as a long-term illness, your GP may need to carry out some tests to diagnose the cause.

Your GP may refer you to a paediatric specialist for further assessment and tests. These may include blood tests to check for any problems with your hormones. Ultrasound and MRI scans may also be used to check for tumours and the functioning of glands and organs.

As with early puberty, treatment for late puberty will depend on the underlying cause. In most cases, treating the underlying causes should trigger puberty. In some cases, you may need to take medicines containing hormones to trigger the start of puberty.

Puberty describes the time in life when the body matures sexually and the reproductive organs become functional.

It’s caused by a release of the sex hormones testosterone and oestrodiol in the body.

Testosterone is the male sex hormone that’s produced by the testis (the male sex organs). Oestrodiol is the main female sex hormone that’s produced by the ovaries.

Puberty causes a number of changes to occur which can be categorised as:

• physical changes – including rapid growth spurts, the development of breasts in girls and an increase in penis size in boys
• psychological changes – which can cause teenagers to become moody, self-conscious and aggressive
• behavioural changes – which can cause some teenagers to experiment with new and potentially risky activities, such as smoking, drinking, alcohol and sex

Read more about the symptoms of puberty and the causes of puberty.

When does puberty start?

There’s no set age when puberty starts. The age at which puberty begins and the rate of development differs between individuals.

Most girls begin puberty at 8-14 years of age, with 11 the average age. Girls develop quicker than boys. Most girls reach full sexual maturity within four years of starting puberty.

Boys tend to develop later than girls. Most boys begin puberty at 9-14 years of age, with 12 the average age. Most boys reach maturity within four years of starting puberty.

Early or delayed puberty

If a child experiences puberty earlier than normal it’s known as early or precocious puberty. Delayed puberty is where puberty occurs later than normal.

In some cases, early or delayed puberty may be caused by an underlying condition. If there’s no obvious cause, such as a long-term illness, tests may be needed to help diagnose any problems.

Read more about the complications of puberty.

The physical changes that occur during puberty are usually marked by distinct stages of development.

The stages are known as Tanner stages, named after Professor James Tanner, the child development expert who first identified them.

The Tanner stages give average ages of development, although there can be significant variation among children and teenagers. You therefore shouldn’t worry if you reach a stage of puberty before or after your friends do.

Tanner stage one

Tanner stage one describes the body before the onset of puberty. This period is sometimes referred to as pre-pubertal.

Tanner stage one isn’t associated with any particular ages, and there aren’t any significant physical changes in girls or boys.

Tanner stage two

In girls:

• usually occurs at around 11 years of age
• breast buds develop – they’re often very tender during the early growth stages (this is normal); one breast bud may also start to develop many months before the other one – again this is normal and the breast tissue will even up over the course of puberty
• your areola (area of skin surrounding the nipple) will begin to swell
• pubic hair will start to develop along the labia (lips of the entrance to the vagina)
• the womb becomes larger in response to oestrogen
• you’ll grow taller by 7-8cm (2.8-3.2 inches) a year

In boys:

• usually begins at about 12 years of age
• your scrotum (the pouch containing the testes) will begin to thin and redden; your testicles will increase in size
• fine pubic hair will start to appear at the base of your penis

Tanner stage three

In girls:

• usually occurs after the age of 12
• the tissue beneath the areola continues to grow and spread out to provide the fullness of your breast; you may need to buy your first bra
• your pubic hair will become coarser and curlier and you’ll begin to grow underarm hair
• you may develop spots (acne, see below) on your face and back
• you’ll grow taller by an average of 8cm (3.2 inches) a year – the highest growth rate

In boys:

• usually occurs after the age of 13
• your penis will grow and lengthen, and your testicles will continue to grow
• your pubic hair will become thicker and curlier, spreading to the soft mound of skin above your genitals
• your breasts could swell slightly due to the growth of breast tissue; about a third of teenage boys have some breast tissue growth which usually settles down after a few years (read more about ‘man-boobs’)
• you may begin to have ‘wet dreams’ – involuntary ejaculations of semen during sleep
• your voice should ‘break’ (the pitch and tone of your voice may start to suddenly change for short periods of time)
• the size of your muscles will increase, and you will grow taller by 7-8cm (2.8-3.2 inches) a year

Tanner stage four

In girls:

• usually occurs at the age of 13
• your breasts slowly develop into a more adult shape, with your nipple and areola swelling to produce a secondary mound above the level of the breast (this will disappear after the rest of your breast has developed)
• your pubic hair will start to look more adult-like in appearance but won’t have spread to your inner thighs
• you’ll usually have your first period and you should start having regular periods in 6-12 months of your first one (around 10% of girls start their periods during stage five)  
• your growth rate will begin to slow down, growing taller by an average of 7cm (2.8 inches) a year

In boys:

• usually occurs at around 14 years of age
• your penis and testicles will continue to grow, and your scrotum will become darker
• your pubic hair will appear more adult-like, but won’t have spread to your inner thighs
• you should start growing underarm hair
• your voice will change permanently
• you may develop acne

Tanner stage five (final stage)

In girls:

• usually occurs at just over 14 years of age
• your breast becomes adult-like in shape
• your pubic hair should spread to your inner thigh
• your genitals should have fully developed by the end of this stage
• you’ll have stopped growing and reached your adult height 1-2 years after your periods started; from the age your periods start, you’ll have another 5-7.5cm (2-3 inches) of growth in height

In boys:

• usually starts at about 15 years of age
• your genitals will look like an adult’s, and pubic hair will spread to your inner thigh
• you’ll begin to grow facial hair and may have to start shaving
• your growth should slow down and you should stop growing at around 16 years of age (but your muscles may continue to grow)
• most boys will have reached full adult maturity by 18 years of age

Acne

During puberty, your body becomes more sensitive to the hormone testosterone, which is present in both boys and girls. Testosterone causes small glands in your skin to produce too much oil (sebum).

Dead skin can also block the opening of hair follicles (the small tubes in your skin that hold a hair in place). The sebum can build up behind the blocked follicle, which can cause spots (blackheads or whiteheads) to develop.

Hormonal changes also alter the levels of acid in your skin, encouraging the growth of bacteria. If bacteria infect a blocked hair follicle, a deeper infection can occur, such as a spot (pustule) or nodule.

Mild to moderate acne can usually be treated with antibacterial cream. In more severe cases, your GP may recommend antibiotic tablets.

Read more about acne.

Body odour

During puberty, your body develops large sweat glands around your armpits, breasts and genitals. These are known as apocrine glands.

Apocrine glands release sweat in response to stress, emotion and sexual excitement. In some cases, the excess sweat can cause body odour.

Periods

A girl’s monthly periods usually start between 11-14 years of age (usually at 12-13). They continue until the menopause, which usually occurs around the age of 50.

In the days leading up to your period, you may have a number of symptoms, including:

• sore breasts
• irritability
• backache
• spots
• feeling very emotional or upset

These symptoms should pass once your period starts. Many girls and women have pain or cramping in their abdomen (tummy), back and vagina. This is often referred to as period pain. Taking paracetamol may help to relieve period pain.

Psychological and behavioural changes

Puberty can often be a particularly difficult time. You’re forced to cope with changes in your body and possible side effects, such as acne or body odour, just at the time when you feel self-conscious about your body and self-image.

Puberty can also be an exciting time, as you develop new emotions and feelings. However, the ’emotional rollercoaster’ experienced during puberty can have psychological and emotional effects such as:

• unexplained mood swings
• low self-esteem
• aggression
• depression 

These feelings can be a normal part of growing up and going through puberty. But if they’re having a serious impact on your life, you may wish to talk to someone close to you, such as a friend or relative.

You could also ask your GP, or you could contact a support organisation, such as ChildLine, for free and confidential help and advice. Their helpline number is 0800 1111, or you could contact them through the ChildLine website.