Puerile
Part of speech: adjective
Definition: Characteristic of, or pertaining to, a boy or boys; confer: puellile. Childish; trifling; silly.
Immature
Part of speech: adjective
Definition: Not fully formed or developed, unripe.Childish in behavior.
Example sentence: Humanity has advanced, when it has advanced, not because it has been sober, responsible, and cautious, but because it has been playful, rebellious, and immature.