The hormonal changes that characterize puberty, which usually begins in 10-11 years of age in girls and in 12-13 years of age in males. The development pattern is very different, but diversity can usually be regarded as normal.
A young person's development is influenced by many different factors. Usually the girls begin to develop earlier than boys, and development patterns are also influenced by hereditary factors - if a mother got her first menstrual period relatively late, it is likely that her daughter also developed late. Similarly usually boys development to follow his father's pattern.
The general health condition is also important when it comes to sexual development. Malnutrition and disease in childhood can delay puberty, and children who are smaller and thinner than average, often developed late.