Generally speaking, the human body starts to have a sexual and reproductive function from the age of 10 or so, and the sexual function is at its peak at the age of 20-30, and gradually decreases after the age of 30. If a person’s sexual function gradually decreases before the age of 30, it is a sign that the human body’s sexual function is beginning to age. However, sexual function varies greatly among individuals, with some already experiencing impotence at the age of 30 and others still having a sex drive at the age of 80, so sexual decline actually begins at the age of 30.