Do under 18's go on Adult 'hook-up' sites like 'Cupid' and 'Adult Friend Finder'?

I mean under-18's as in 15,16 and 17. Are there alot of under-18's on these sites? Why is it illegal to go on these sites under 18 years, yet you can have sex at a much younger age. Please do not post comments like 'I am sure they do' and things like that.