Antonyms for Immature (opposite meaning)
WordNet sense 1 ((used especially of persons) having lived for a relatively long time or attained a specific age):
oldWordNet sense 2 (fully developed or matured and ready to be eaten or used):
mature, ripeWordNet sense 3 ((of birds) having developed feathers or plumage; often used in combination):
fledgedFrom the ODE community, based on WordNetadd/edit