Young people go into several physical and mental changes while moving from childhood towards adulthood. The stage between these two is adolescence. It starts at around 12 years and last till 16 to 17 years. The changes that take place at this time is different in a boy and a girl. Changes in a boy:
Growth of whole body
Voice change
Growth of body hair and pubic hair
Growth of testicles and scrotal sac
Growth of facial hair surrounding the upper and lower lips