immature

ephebic, minor, preteen, subadult, teenage (or teenaged), underage

embryonic, emergent

callow, green, inexperienced, puerile, raw

babyish, childish, childlike, infantile, infantine, kiddish

undeveloped, unfinished, unfledged, unformed, unripe, unripened

blooming, blossoming, burgeoning, flourishing, flowering

boyish, brattish, bratty, girlie (or girly), girlish

childlike, innocent, naive (or naïve), simple, simplistic, unsophisticated

babyish, childish, infantile, infantilized, infantine

boyish, girlish, juvenescent, kiddish, young, youngish, youthful

maidenly, virginal

ingenuous, innocent, naive (or naïve), tender

unknowing, unseasoned, unsophisticated, untrained, untried