WordNet sense 1 (suggestive of youth; vigorous and fresh):

vernal, young, youthful

WordNet sense 2 (young and immature):

tender

WordNet sense 3 (being of the age 13 through 19):

adolescent

WordNet sense 4 (not fully developed or mature; not ripe):

green, unripe, unripened

WordNet sense 5 (indicating a lack of maturity):

childish

WordNet sense 6 (displaying or suggesting a lack of maturity):

jejune, juvenile, puerile

WordNet sense 7 (not yet fully formed):

unformed

WordNet sense 8 ((of birds) not yet having developed feathers):

unfledged

WordNet sense 9 (not developed):

undeveloped