Occurs usually between ages 8 and 14. For girls, the sequence of biological changes are as follows: 1) growth of nipples, 2) first pubic hairs, 3) height spurt, 4) first menstrual period (menarche), 5) final pubic-hair growth, 6) full breast development. Average age of menarche is 12 1/2 years. For boys, the sequence is as follows: 1) growth of testes, 2) first pubic hairs, 3) enlargement of penis, 4) first ejaculation of seminal fluid (spermarche), 5) height spurt, 6) final pubic-hair growth. Average age of spermarche is 13 years. For both, puberty is usually completed in about 4 years.