Big difference between 13-15 though. Wouldn't bat an eyelid if it were 16 and 15.
16 and 13 is a Year 11 and a Year 8.
But why? 15 is just as illegal as 13. You cannot say 15 is fine, but 13 is not - it is either legal or not.
Some countries, the legal age is 13... Looking around England these days, you'd probably be surprised at how many 13 year olds are sexually active imo.
Not sure about others but I was kind of thinking of it in terms of morals rather than laws