| Girls | Average Age | Boys | Average Age |
|---|
| Ovaries increase production of estrogen and progesterone. | 9 | Testes increase production of testosterone. | 10 |
| Internal sex organs begin to grow larger. | 9 1/2 | External sex organs begin to grow larger. | 11 |
| Breast development begins. | 10 | Production of sperm and first ejaculation | 13 |
| Peak height spurt | 12 | Peak height spurt | 14 |
| Peak muscle and organ growth, including widening of hips | 12 1/2 | Peak muscle and organ growth, including broadening of shoulders | 14 1/2 |
| Menarche (first menstrual period) | 12 1/2 | Voice lowers. | 15 |
| First ovulation (release of fertile egg) | 13 1/2 | Facial hair appears. | 16 |
| Source: Data from Brooks-Gunn & Reiter (1990). |