Although referred to as the teen years, adolescence can and often does begin earlier, especially in girls. It can end as late as the early twenties. Many significant events occur during this time, like learning to drive, feelings of sexual intimacy, completing high school, and entering college. Sex organs (breasts and testes) will develop, and height, weight, and muscle mass will all increase. 

During adolescence, many physical and emotional changes will occur in both boys and girls.  Suddenly the way you look, the clothes you wear, and what your friends think of you will become more important than ever before. Friends quickly take the place of your parents as the most important people in your life.