(now rare) Occurring before the proper time; untimely, premature (especially of death).
Not fully formed or developed; not grown.
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.