We are going slightly off topic here but you deserve a response.
Visit the
Child Sex Abuse page on wiki and see for yourself the damage done by sexual acts upon children.
The real problem we have here is defining 'children'. I have spent a lot of my adult life working with kids, many of them around the age of puberty; 10, 12, 13, and so on. None of the thousands of kids I've worked with have been known to engage in sexual acts with each other just because the girls are menstruating and the boys have started to grow hairs in certain places.
If they aren't doing this between themselves at this age it clearly shows they aren't ready by nature, and I may be mistaken when I say that it seems the same in most cultures and countries (except for a few, desperate parts of the Earth), then what makes it any different from an adult performing sexual acts with a child?
I grew up in a pretty low life, immoral community and even there only a tiny percentage (perhaps 1% at a guess) of girls and boys were getting it on under the age of 15/16. The majority were not.
I don't have all the answers and I can only speak from experience within a few different cultures of a few different religions as to what I personally know on the topic. The difficulty is defining what a child is and even then I think that determining whether a person is a child or adult is more based on the person as an individual and not the number that denotes their age, as you said yourself.