At the age of 13, boys have entered the middle of puberty, which is the age when secondary sexual characteristics appear. The earliest development of boys is the development of testicles, which is the first sign of puberty. The androgens secreted by testicles can promote the appearance of secondary sexual characteristics, which include the growth of pubic hair in boys and gradually become darker, thicker and curlier, penis enlargement and thicker, testicles development and enlargement, neck throat knot begins to protrude, speaking voice becomes louder and thicker, and semen secretion. At the same time, height and weight also grow rapidly, bones become hard and muscles develop.