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.