During childhood, girls and boys are physically very alike. Although boys are on the avarage one centimetre taller than girls at this age, girls are slightly more mature than boys. Their bone age is about three weeks older. At about ten years the average height of girls is still slightly less than the average height of boys, and the boys tend to be a little stronger than the girls.
Between the ages of 10 and 12 these slight differences between the sexes are totally reversed and become very marked. A mixed class of 11- or 12-year-olds can contain girls who look like young women, towering over boys who are indisputably still children. The girls, whose growth spurt begins at about 10, two years earlier than boys, quickly overtake their male contemporaries in both height and weight. The difference in weight is due partly to the increase in height, partly to an increase in body fat. Not until the age of about 14, when the boys are in their growth spurt and the girls’ is almost over, do the boys regain their physical advantage.
The events of puberty always occur in the same order, although the age at which the changes take place varies widely. Girls mature earlier than boys; their first signs of puberty usually appear between the ages of 9 and 10. The face and hips become more rounded, the nipples more prominent, and the breasts begin to swell. Between the ages of 10 and 11 the sudden acceleration in growth known as the growth spurt of adolescence begins. A girl may grow as much as 10cm in a year, and this rapid growth may make her feel clumsy and unco-ordinated. In school-age boys, the first puberty changes do not usually begin until 11 or 12, when the testes and then the penis start to enlarge and the first, fine pubic hair appears.