Boys and girls develop at slightly different paces and in slightly different ways. Girls grow faster early on, and are often bigger than boys throughout elementary school. They mature sooner, hitting their adolescent growth spurt an average of 2 years sooner than boys.
However, once puberty begins, boys quickly catch up to the girls, equaling and then surpassing them in both height and body weight. At the end of the puberty growth spurt, boys are an average of 4 inches (10 cm) taller than girls. By the time they are fully grown, males average a height of around 5 inches (13 cm) taller than females.
It’s also important to note that males and females have slightly different body compositions. Females average around 23% protein versus 25% fat, whereas boys are 40% protein to 15% fat. In other words, boys have more muscle, whereas girls have more body fat (presumably to ensure a healthy store of calories during pregnancy).
This also leads to slightly different nutritional requirements, since muscle requires more calories to maintain, even at rest. “By the time kids reach ages four through eight, boys and girls have slightly different calorie and nutritional needs,” says Dr. Alan Greene (2009, p. 249). Boys will need a bit more calories and a little more protein.
During the prepubescent years, there’s little difference between boys and girls in terms of size, strength, or physical ability. Hormonal differences are also much smaller. So there’s little reason to segregate sports in elementary school, and parents should relax a bit when it comes to gender nonconforming children or concerns about boys and girls playing on the same teams. These size and strength differences really only emerge in earnest when children hit puberty, at which point gender differences do give a discernible competitive advantage to the boys.
Gender differences and the physical disparities they bring can also lead to discrepancies between boys and girls when it comes to injuries. For example girls are more prone to the risk of concussions (in sports as well as in everyday life), presumably because they have weaker neck muscles supporting their head, leaving them more vulnerable to the whiplash motion that leads to concussions. This doesn’t mean girls shouldn’t play sports or that you should treat them like delicate snowflakes; it’s merely an example of how physical differences manifest in children.