puerile

puerile \PYOO-uhr-uhl; PYOOR-uhl\, adjective:

Displaying or suggesting a lack of maturity; juvenile; childish.

And, in one of the most puerile episodes of his adult career, he punishes his old schoolmates for being rich and vulgar by breaking into their houses to soak the labels off their boasted wine collections.
-- Thomas R. Edwards, "Mordecai Richler Then and Now", New York Times, June 22, 1980

Political argument is becoming a puerile cartoon about the moral . . . doing battle with the immoral.
-- George F. Will, "The Costs of Moral Exhibitionism", Washington Post, April 15, 2001

Puerile comes from Latin puerilis, from puer, "child, boy."