there are two diffinitive things that govern whether things are right or wrong:
a-age gap, if a 18 year old was going out with 15/16, it would be seen as acceptable, but even 20 with 16 is seen as a strech. fortunatly that only works from male to female, otherwise me and mf g/f would be screwed...
b-maturity, i know 13 year old girls who can hold down an argument with a 60 year old philosiphy teacher, and 19 year olds who get confused by shiney things. if a girl is mature enough to understand how things will affect her e.t.c then things do change than if she's just playing with barbie.
although it's probably been said before, those two things will usually govern everyone who isn't a townie/rude boy/pikey decisions.
it's all relevant, and depending on the type of relations, if it's just holding hands, then most people would accept a lot younger, but if sexual relations of any kind are involved, then things get a lot more tricky.