It is possible for boys to produce sperm at the age of 13. The age of onset of puberty in boys is 11-13 years. The sequence of pubertal development in boys is the enlargement of testes, increase in scrotal skin folds and deepening of pigmentation, growth and thickening of the penis, growth of pubic hair, axillary hair, and beard, as well as voice becoming lower, sperm production, increase in muscle volume, and decrease in subcutaneous fat. Puberty is the transition period from childhood to adulthood, a process of sexual maturation caused by a series of endocrine changes and the development of reproductive capacity, as well as a process of physical, mental and emotional development. The age at which puberty begins and ends for girls is about two years earlier than for boys, and there is a wide variation in the age at which puberty enters and ends, which can vary from 2-4 years. The physical growth of children accelerates again during this period, with a second peak. At the same time, the development of the reproductive system accelerates and matures, and puberty is marked by sexual development.