While primary sex characteristics are those that are present at birth, secondary sex characteristics are those that appear during puberty. These secondary sex characteristics are caused by hormones released at the time of puberty, which usually is around two years earlier in girls than in boys. While both boys and girls grow taller in their teenage years, the male body becomes more muscular, and the shoulders grow broader than the hips, while the female's hips become wider than her shoulders, and breasts develop.

Some of the first changes in a boy are the growth of his testicles and growth of pubic hair. Later, the chest becomes larger, hair grows in the armpits, muscles grow in the arms and legs and shoulders become larger and stronger. In some males, hair also grows on the chest, but generally, Asian men are less hairy and less muscular than Caucasian men, and those from Africa have coarser body hair. Facial hair, which usually grows first above the lips and later grows on the cheeks, may grow into a mustache and beard unless the boy shaves regularly. The larynx (voice box) becomes larger as well, resulting in a deeper voice.

In girls, pubic hair begins to grow, followed by underarm hair; breasts develop, with the areola around the nipple becoming darker. Fat deposits around the hips and buttocks also contribute to the female's more rounded appearance. Although the menstrual cycle begins, it may be irregular, with some monthly cycles occurring without ovulation. Since 1840, menstruation has been beginning a few months earlier every decade, possibly due to better nutrition or to the consumption of meat that contains hormones. In general, girls gain less height and weight than boys do during their teenage years.