What happens in puberty?

Before puberty, there are hardly any differences between girls and boys. Their bodies, faces and voices are similar. The biggest differences are in their external sex organs (the visible genitals) – although they aren’t very developed at this stage.

It is still not clear what actually triggers puberty. Girls start puberty at around age nine. Boys don’t start until they’re around eleven. When puberty begins, the child’s brain sends signals to start the production of sex hormones. At first, these “chemical messengers” are made in the adrenal glands. The hormones then trigger gradual changes in the child’s body. One of these changes is the development of the reproductive glands (gonads). In boys, these are the testicles (commonly called "balls" or "nuts"). In girls, they are the ovaries. Later, the sex hormones are made in the gonads, and then larger amounts are released into the bloodstream.

Puberty ends in stages: Most girls reach sexual maturity by the age of 14; most boys by the age of 16. Girls’ bodies normally stop growing around the age of 16. Boys have usually finished growing by the time they’re 19. But it can take up to the early twenties for the last physical changes to happen. This can even take a little longer in boys.

The increase in the amount of sex hormones at the start of puberty triggers a growth spurt. It is completely normal for teenagers to grow by 5 to 8 centimeters (about 2 to 3 inches) in one year at this stage. For a while, girls are often taller than boys their age because they start puberty earlier. Once the gonads start making the hormones, the transition to adulthood is mainly regulated by the estrogen hormones made in girls' ovaries and the testosterone made in boys' testicles.

Girls stop growing earlier than boys because the estrogen slows down their growth when greater amounts are made. That is why adult women tend to be a little shorter than adult men. Their bone proportions are also slightly different. Women’s hips are usually wider than their shoulders are.

As children and teenagers grow, their bones become longer and heavier. Their voice box (larynx) grows too. The bigger it becomes, the deeper their voice becomes. Boys’ voice boxes grow larger than girls’ voice boxes, which is why men’s voices usually sound deeper than women’s voices. While the voice box is still growing, the voice may sound shrill or croaky, and sometimes jump between high and low sounds. This is what people mean when they say someone's voice is “breaking.” It is generally more noticeable in boys than it is in girls.

As well as the gonads, the other sex organs develop at this stage too. A boy’s penis and a girl’s labia and clitoris grow bigger. But these primary sex characteristics don't only grow in size. They also start to develop all the functions needed to make babies. Boys start to ejaculate – often while they’re asleep (also known as "wet dreams"). In girls, the womb develops into a fully functional organ, preparing their body for possible pregnancy once a month. This leads to the girl’s first period around the age of 13. Girls’ ovaries also get ready to release a fertile egg every month. But it can still take quite some time until everything is ready and the girl can have babies.

Apart from having different body sizes and different primary sex characteristics, adult women and men have different secondary sex characteristics, too. One of these characteristics is the female breast. During puberty, special "milk glands" inside the girls’ breasts (called mammary glands) start to grow, and more body fat is stored in the breast tissue. As a result, their breasts gradually develop. The female hormones also cause the skin in other parts of the body to be “padded out” with more fatty tissue. Together with women’s typical bone structure, this generally leads to a female-looking body shape.

One of the other secondary sex characteristics is how much hair grows, and where. Even before puberty, children have hair almost all over their bodies. But – apart from the hair on their head – it tends to be very fine, light and short, so you can hardly see it. During puberty, the hair structure changes in some parts of the body. The hair in the pubic areas and armpits of both girls and boys becomes darker and thicker. It is usually curly and can be several centimeters long. Boys’ pubic hair grows more up toward the belly button. Many boys grow dark hair on their legs and, later, their chest. The male sex hormone testosterone causes hair to grow on their face, too.

Hormones also make the skin produce more oil, which is why a lot of teenagers get acne ("spots"). Once their hormone levels have settled down (typically by their early twenties), acne usually goes away on its own.

We’ve all heard about teenagers' mood swings. These emotional ups and downs are probably caused by changes in the brain. Teenagers' moods can “swing” between sadness and extreme happiness. Sometimes, they behave maturely and “sensibly,” only to act silly and childish a moment later. Often there are rebellious phases to cope with, when the teenager has fits of anger, is generally irritable and makes demonstrative attempts to distance themselves from their parents. These phases can lead to conflicts at home and, occasionally, in friendships. As well as having problems with those around them, some girls and boys have their own personal problems. This emotional rollercoaster is intensified by their first romantic feelings, yearnings and sexual desire. Often, their bodies have already stopped changing by the time they become emotionally and sexually well-balanced.