From The Collaborative International Dictionary of English v.0.48 [gcide]:
grownup \grownup\ adj.
fully developed; adult; mature; -- of people and animals; as,
Act like a grownup!.
Syn: adult, big, full-grown, fully grown, grown.
[WordNet 1.5]
From WordNet (r) 3.0 (2006) [wn]:
grownup
adj 1: (of animals) fully developed; "an adult animal"; "a grown
woman" [syn: adult, big, full-grown, fully grown,
grown, grownup]
n 1: a fully developed person from maturity onward [syn:
adult, grownup] [ant: juvenile, juvenile person]