I don't know about the UK, but in the US, once a person reaches a certain age, restrictions are applied but exceptions are also made.

For example, in my state... when you're 17-18, you can't date anyone under 16. But you can date someone 16-17. When you're 16, you can be with someone under 16 but only to the age of 13. It doesn't imply that the youngest age is 13, but my state defines a child as one who has not reached puberty, which means menstruation/etc here. Not the onset of puberty, which begins at 8-10. It's really confusing.

Like Traci said, I suggest doing more research.

My state isn't a good example to go by. I'll list two more.

In Connecticut, the age of consent is 18. But there are four exceptions made. A relationship is legal between someone who's 12-13, 13-15, 15-17, 16-18. In Florida, the age of consent is 18, but a seven year age difference is legal no younger than 16. So a 16 year old can date a 23 year old, but a 15 year old can't date a 22 year old.