Synonyms for Immature:

adj.

adolescent (adjective)
juvenile, youthful, puerile, pubescent, adolescent, teenage.
babyish (adjective)
infantile.
green (adjective)
unfledged, green.
incomplete (adjective)
fractional, embryonic, scanty, patchy, insufficient, missing, partial, unachieved, sectional, unfinished, undone, unfulfilled, short, unattained, lacking, Unaccomplished, deficient, discontinuous, fragmentary, Curtailed, segmental, incomplete, unsatisfactory.
juvenile (adjective)
infantile, young, childish, pubescent, juvenile, adolescent, youthful, junior.
rudimentary (adjective)
larval, embryonic.
young (adjective)
callow, puerile, young, child-like, youthful, unripe, junior, fresh, girlish, pubescent, inexperienced, adolescent, boyish, blooming, childish, juvenile, teenage, green.
young, inexperienced (adjective)
callow, jejune, green, undeveloped, juvenile, tender, baby, adolescent, underdeveloped, unformed, childish, unfinished, unsophisticated, youthful, unripe, babyish, unfledged, half-grown, infantile, puerile, Sophomoric.

n.

insane, ludicrous, silly, laughable, crazy, ridiculous, over-the-top, mad, not in your right mind/not right in the head. immature (noun)
childlike, undeveloped, newborn, vulnerable, preadolescent, larval, babyish, jejune, childish, unfeathered, unripe, youngish, boyish, teen, prepupal, boylike, naive, green, teenage, junior, tender, vernal, unripened, small, schoolboyish, prepubertal, callow, youthful, puppyish, adolescent, schoolgirlish, inchoative, girlish, unfledged, preteen, early, inexperienced, infant, pupal, prepubescent, embryonal, pubescent, juvenile, unformed, unaged, teenaged, embryonic, unsophisticated, puppylike, puerile, little, infantile, baby, underdeveloped, formative, childly.
youthful (noun)
half-grown, Sophomoric.

Other synonyms:

full-blown, early. full-fledged. growth.

Usage examples for immature

  1. If he saw nothing in it now but the immature efforts of an amateur! – The Shadow of the East by E. M. Hull
  2. This is the newest- looking place we have yet visited, the youngest in appearance in this young world; and I have experienced to- day a disagreeable instance of its immature civilization, or at any rate its small proficiency in the elegancies of life. – Records of a Girlhood by Frances Ann Kemble