Precocious

[Sarah]

Matured early.

[Donald Trump]

Unusually advanced or mature in development, especially mental development.

[Sonia]

1.(of a child) Having developed certain abilities or proclivities at an earlier age than usual
2.(of behavior or ability) Indicative of such development
3.(of a plant) Flowering or fruiting earlier than usual

[Justin Bieber]

How you sound when you say Supercalifragilisticexpealidocious.

[Jason Yan]

Ripe or mature before the proper or natural time; early
or prematurely ripe or developed; as, precocious trees.

[Kai]

A [kid] who hit [puberty] by [six].

[Gregory]

[A guy] or girl [that hits] [puberty] at 6.