WordNet sense 1 (any new participant in some activity):

entrant, freshman, neophyte, newbie, newcomer, starter, fledgeling

WordNet sense 2 ((of birds) having developed feathers or plumage; often used in combination):

mature, fledged

WordNet sense 3 (young and inexperienced):

callow, unfledged

WordNet sense 4 (lacking practical experience or training):

inexperienced, inexperient