precocious
adjective 
(of children) showing unusually early mental development or achievement, or esp. disapproving behaving as if you are much older than you really are 
The child's precocious ability to play chess is astonishing.
At the precocious age of 29 she was made a professor of philology.
ESPECIALLY DISAPPROVING He's such a precocious little brat that none of the other children want to play with him.

precociously
adverb 
He was a precociously gifted boy who went to university at the age of 14.
She decided on a theatrical career at a precociously young age.

precociousness, formal precocity
noun [U] 
What is surprising about the new generation of writers is their precociousness, for they all began to write at a very young age.