Puberty is the time in life when a boy or girl becomes sexually mature. It is a process that usually happens between ages 10 and 14 for girls and ages 12 and 16 for boys. It causes physical changes, and affects boys and girls differently. In girls: 1. The first sign of puberty is usually breast development. 2. Then hair grows in the pubic area and armpits. 3. Menstruation...