Childish, a.
1. Of, pertaining to, befitting, or resembling, a child. "Childish innocence." Macaulay.
2. Peurile; trifling; weak. Methinks that simplicity in her countenance is rather childish than innocent. Addison.
Note: Childish, as applied tc persons who are grown up, is in a disparaging sense; as, a childish temper.
Shades of meaning
Fernald has no entry of its own for childish; he lists it under youthful.
Where he writes about it:
Masculine may apply to the distinctive qualities of the male sex at any age; virile applies to the distinctive qualities of mature manhood only, as opposed not only to feminine or womanly but to childish, and is thus an emphatic word for sturdy, intrepid, etc. — from masculine
Boyish, childish, and girlish are used in a good sense of those to whom they properly belong, but in a bad sense of those from whom more maturity is to be expected; childish eagerness or glee is pleasing in a child, but unbecoming in a man; puerile in modern use is distinctly contemptuous. — from youthful