When it starts
For girls, the average age is 11, and anywhere between 8 and 13 is ordinary. For boys, the average is 12, and anywhere between 9 and 14.
The physical changes take around four years to work through. The emotional side takes longer, and does not finish neatly at the same time as the growing.
Boys start roughly a year later on average than girls, which is why a Year 7 or Year 8 class contains children at wildly different stages, with the girls often a head taller. That gap closes entirely, and it predicts nothing whatever about eventual height or build. Starting early and starting late both land in the same place.
The order things happen in
Timing varies enormously between individuals. Sequence barely varies at all, which makes it the more useful thing to know.
In girls, breast development is usually the first sign — often one side before the other, which is normal and which causes a certain amount of unnecessary alarm. Pubic and underarm hair follows, along with more sweating and spots. The growth spurt comes early in the process. Body shape changes, with the hips widening, the waist narrowing and fat laid down on the arms, legs and upper back. Periods arrive last, generally about two years after the first signs appeared.
In boys, the first sign is the testicles getting larger and the scrotal skin becoming thinner and redder. This is worth stating plainly, because it is the change nobody mentions and it is almost invisible — which is why boys well into puberty often believe nothing has started. Penis growth follows, then pubic hair, then body and facial hair, which thickens gradually over years rather than arriving. The voice deepens, usually cracking and jumping about on the way. The growth spurt comes late.
Growth
At the peak of it, girls gain roughly five to eight centimetres a year and boys rather more. Hands and feet often go first, which is why shoes stop fitting before anything else does, and coordination lags behind the new proportions for a while — the clumsiness of this age is real and temporary.
The difference in timing between the sexes has one practical consequence worth knowing. A girl's growth spurt happens early in puberty and is largely finished by the time periods start or shortly after. A boy's comes late, so a boy who is short at fourteen may have most of his growing still ahead of him. The two situations look similar and are not.
The changes nobody warns anyone about
Most of the distress in puberty comes from changes that are entirely normal and entirely unexpected.
- Body odour. New sweat glands become active, and the smell is produced by hormones rather than by poor washing. Washing and deodorant are still the answer; being told it is a hygiene failure is both wrong and unhelpful.
- Spots. Also hormonal rather than a matter of dirt. Scrubbing makes them worse. A pharmacist is the right first stop and can supply things that work; a GP is worth seeing where acne is severe, painful or leaving marks, because effective treatment exists and scarring is avoidable.
- Breast tissue in boys. Temporary swelling and tenderness behind one or both nipples is common in boys going through puberty, and it settles on its own. It is worth saying out loud, because a boy who does not know this is happening to other boys tends to conclude that something is seriously wrong and say nothing.
- Erections and wet dreams. Spontaneous, frequently unrelated to anything, and ordinary.
- Vaginal discharge. A clear or whitish discharge usually begins some months before the first period, and is a reasonable sign that one is coming rather than anything to treat.
- Asymmetry. One breast developing before the other, one testicle sitting lower or being slightly larger. Both normal.
- Weight. Putting on weight is part of puberty rather than a failure of it. The change in body shape involves laying down fat as well as growing, and that is what is supposed to happen. This is also the stage at which comparison and self-criticism commonly begin, and distress about the way a changing body looks is worth taking seriously early rather than waiting for it to pass.
Sleep, which shifts for a reason
At around thirteen, the body clock moves later. A teenager who cannot get to sleep at ten and cannot get up at seven is not necessarily being difficult.
The evidence that this is more than habit is reasonably good. Later bedtimes persist at weekends and through the school holidays, when nothing external is setting a wake time. The shift towards being an evening person appears at about thirteen, peaks at roughly twenty and then reverses, and the pattern turns up across different countries and cultures. Through adolescence, sleep on school nights falls by something like fourteen minutes for each year of age, and the debt accumulated over the week gets partly repaid with around two extra hours a night at the weekend.
What is not settled is the mechanism. Whether it comes from a shifted internal clock, a slower build-up of sleep pressure through the day, or some combination, remains genuinely uncertain, and real behavioural pressures — homework, friends, phones, less supervision of bedtimes — contribute as well. The honest summary is that the pattern is well established and the explanation is not, which matters mainly because it rules out the two extreme readings: that it is pure laziness, and that nothing can be done about it.
The parts that respond to effort are the evening light and the weekend. A wake time that swings by four hours between Friday and Sunday puts the clock back where it started every week.
Periods
Most start at around 12, with anywhere from 8 to 15 within the normal range.
What is ordinary at first. For the first eighteen months to two years there may not be a period every month. Skipped months, two in one month and ones that arrive late are all usual. After about two years, cycles generally settle to something between 25 and 35 days. A period itself typically lasts three to seven days, with pads needing changing three or four times a day on the heavier ones.
What is heavier than it should be. Soaking through pads, needing to change every couple of hours, needing a pad and a tampon at the same time, or soaking bed sheets overnight. Heavy periods at this age are treatable and are a common cause of iron deficiency, so this is worth an appointment reasonably promptly rather than being endured.
Pain. Cramps are normal and usually worst over the first two or three days. Paracetamol or ibuprofen helps, though ibuprofen is not suitable for someone with asthma, a stomach ulcer or a known allergy to it. Pain severe enough to mean missing school is not something to put up with — stronger prescribed options exist and work. Pain that begins several days before bleeding starts, continues after it stops, and does not respond to the usual painkillers should be assessed properly.
Being ready beforehand. Since the first period will not announce itself in advance, products in a school bag and a conversation already had are what prevent the version of this story that gets remembered for thirty years.
Mood, and what it is and is not
Mood swings, irritability, tearfulness, patchy self-esteem and a new appetite for arguing about household rules are all part of the ordinary picture.
So are crushes, including on adults the young person admires. They are common, they are not embarrassing, and they say very little about eventual sexuality. The response to avoid is teasing. There is one exception that is not a matter of tact: where an adult appears to be making use of a young person's feelings, that is a safeguarding concern and worth getting advice on rather than handling privately.
Distinguishing ordinary moodiness from something that needs attention is mostly a question of pattern rather than intensity. Low mood that is persistent rather than fluctuating, loss of interest in things previously enjoyed, withdrawal from friends, and changes in sleeping or eating that run alongside it are worth a GP appointment. So is distress about body changes that is not shifting.
Talking about it in advance
Earlier than feels necessary is about right, because the aim is simply that the first change is not a surprise. In instalments works better than one set-piece conversation, and questions answered as they come are easier than questions invited.
Relationships, sex and health education covers this in school, so no parent has to be the only source, and a school will say what its programme contains. Supplies in the house before they are needed, rather than after, removes one awkward conversation entirely.
Embarrassment at this age is close to universal, and respecting it — privacy in the bathroom, knocking on doors, not discussing any of it in front of siblings — is part of the job rather than an indulgence. Children with additional needs generally need the same information given more explicitly, more concretely and more than once.
When to see a GP
Starting too early. Signs of puberty before 8 in a girl or before 9 in a boy. Early puberty mostly affects girls, where it is often harmless and runs in families; in boys it is considerably more likely to have an underlying cause, which is why it is investigated more thoroughly.
Starting too late. No breast development by 13, or breast development but no periods by 15. No testicular development by 14. More broadly, puberty that has not started by around fourteen in either sex is worth raising.
Assessment typically means blood tests for hormone levels, an X-ray of the hand to estimate likely adult height, and sometimes an ultrasound or an MRI scan to check the glands and organs involved. Where treatment is warranted, medication can pause early puberty for a few years or get a delayed one started, and it is generally offered where the timing is causing real difficulty — a final height likely to be very short, or genuine distress — rather than simply because the dates are unusual.
Also worth an appointment:
- Very heavy or very painful periods, as above.
- Periods that stop for more than three months after having been regular.
- Irregular periods alongside worsening acne, excess facial or body hair, or darkened thickened skin on the neck, which together can point to polycystic ovary syndrome.
- Severe acne, or acne leaving marks.
- Low mood or anxiety that is persistent rather than passing.
Contraception, infections, consent and the HPV vaccination — all of which become relevant in the same few years, and all of which are confidential, including for under-16s — are covered under sexual health for teenagers.