Puerile, a.
Etymology: [L. puerilis, fr. puer a child, a boy: cf. F. puéril.]
Boyish; childish; trifling; silly. The French have been notorious through generations for their puerile affectation of Roman forms, models, and historic precedents. De Quincey.
Syn. -- Youthful; boyish; juvenile; childish; trifling; weak. See Youthful.
Shades of meaning
Fernald has no entry of its own for puerile; he lists it under youthful.
Where he writes about it:
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