At what age is birth control safe to begin?

After menarche: Birth control is safe to begin if used correctly at any time after menarche (that is, the first period). Some girls are prescribed birth control pills due to very heavy periods as early as 10 or 11. This is not to say that a 10 or 11 year olds (or even most 16 year olds) are mature enough to make appropriate decisions about sexual activity.