The answer is puberty, a period when girls and boys become sexually mature. It is a process that usually takes between ten and fourteen years in girls, and between twelfth and sixteenth in boys, although it may be a variation in age. In addition to achieving physical ability to reproduce, puberty also implies visible physical changes on the body. Also, during puberty, the body as a whole grows faster than in any other period of life, except when you were a baby.