a term like jailbait. is that used in UK?.. here in US there are laws concerning "the age of consent" it varies from state to state, but basically a girl under 14 to 16 is considered to young to be able to consent to sex, and even if she give verbal consent, a man can be held for statutory rape. Most often, its is not prosocuted, (speaking for NY here,) unless the man in question is more than 5 years older-- Wait, rereading, i realized i was being sexist, the law applies to both young men and women, not just women!
so a 16 year old guy could get away with sex with a 14 year old (even though she is under age of consent) but a 20 year old guy couldn't.. the courts have held that kids who know each other through school/siblings/are close in age, are not taking unfair advantage, but a adults should know better.

young girls who dress and behave like adults are jail bait.(i have never heard the term applied to a young guy!)