Synonyms for Fully grown:

adj.

mature (adjective)
full-blown, grown, developed, full-grown, full-fledged.
ripe (adjective)
mature, adult.

n.

full-blown, developed, fully fledged, immature, full-fledged, infantile, growth. fully grown (noun)
big, grown, full-grown, grownup, mature, adult.

Other synonyms:

fully fledged. full-blown, infantile, developed. full-fledged. growth. Other relevant words:
fully fledged, full-fledged.