Boyish

[Sarah]

Resembling a boy in a manners or opinions; belonging to a boy;childish; trifling; puerile.A boyish, odd conceit. Baillie.

[Sarah]

Adj. A brilliantly scathing insult or wisecrack used to describe an ultra-annoying female who displays physical characteristics and qualities that are reminiscent of a pre-pubescent 13-year-old boy.

[Justin Bieber]

Of, like, or characteristic of a male child or young man.