Boyish, a.
Resembling a boy in a manners or opinions; belonging to a boy; childish; trifling; puerile. A boyish, odd conceit. Baillie.
Shades of meaning
Fernald has no entry of its own for boyish; 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