Similar word to undergrown:

adolescent, baby, babyish, callow, childish, crude, green, half-grown, imperfect, infantile, infantine, jejune, juvenile, kid, kidstuff, premature, puerile, raw, sophomoric, tender, tenderfoot, underdeveloped, undergrown, undeveloped, unfinished, unfledged, unformed, unripe, unseasonable, unseasoned, unsophisticated, untimely, wet behind ears, youthful