immature

Etymology

From Middle French immature. Partially displaced unripe, from Old English unrīpe (“unripe, immature”).

adj

1. (now rare) Occurring before the proper time; untimely, premature (especially of death).
2. Not fully formed or developed; not grown.
3. Childish in behavior; juvenile.
   You're only young once, but you can be immature the rest of your life.
   The man was immature for throwing a tantrum.
   Wilhelm Stekel - As quoted in The Catcher in the Rye (1951) by J. D. Salinger. The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one.

noun

1. An immature member of a species.
   There are many genera and even families of Brachypylina for which immatures are not yet known, and thus numerous examples of adult convergence and misclassification remain to be revealed: such is the case with Hypozetes. 2001, DE Walter, H Proctor, RA Norton, Acarology: Proceedings of the 10th International Congress, page 51
   While on a walk the next morning I found what looked like a patch of old growth habitat - perhaps somewhere the fires had missed - and to my astonishment saw a female Red-lored Whistler accompanied by an immature. 2005, Sean Dooley, The Big Twitch, Sydney: Allen and Unwin, page 240