I consider prepubescent persons, say under 12 years, to be insufficiently developed mentally and mostly also physically to carry full independent human rights and responsibilities, to which the right to have sex belongs.
But then I consider unrealistically high age legal limits to be against the human rights young people too. If they are physically and mentally ready and winning go to de facto, it is against their human rights to forbid them de jure.
Sexuality is a complex and partly taboo topic, replace it with alcohol for example, maybe you see more clearly my point: I wanted to drink when I was 13, and I also did. The law limited my rights, but really nobody in my friend circles allowed themselves to be restricted by the law. A practical problem was that we were forced to steal all our alcohol, since buying it was illegal.