Precocious

Precocious: Unusually early development, whether it be of intellectual powers, speech, physical traits, etc.

Precocious puberty is the unusually early development of secondary sexual features. The onset of sexual maturation in a girl before age 8 or a boy before age 9 is usually considered to be precocious puberty.

"Precocious" is to be distinguished from "precocial" which means "capable of a high degree of independent activity from birth" and is usually applied to the young of other species such as certain birds such as ducks.