From WordNet (r) 2.0 (August 2003) [wn]:
grownup
adj : (of animals) fully developed; "an adult animal"; "a grown
woman" [syn: adult, big, full-grown, {fully
grown}, grown]
n : a fully developed person from maturity onward [syn: adult]
[ant: juvenile]
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]