You're making the mistake of thinking that dating is synonymous with intimate relationships. Pre-puberty; when kids talk about having a 'date', they're not thinking of it in the same context as older people would. To them it's hanging out, playing games, watching something on TV, a meal... they're trying to emulate adults, but are mystified by the intimate parts beyond holding hands and (bad) kissing - and as for saying they've 'had sex'; it's an out-and-out lie, the only reason it's said is because they think it makes them seem 'cool'. Most don't even know the first thing about sex, or are completely misinformed by their peers.
Very few will get the facts (though if one gets the truth of the matter it joins in the pool of rumours and misinformation; but it's extremely uncommon to find an individual with all the answers), or even have any drive to find out, until near the end of puberty.
As for the cause of the early sexual/intimate behaviour... the media has been hyper-sexualized for about the last decade and a half. TV, and a lot of other forms, all suffer from the 'sex sells' mentality (what's more sad is that they're right). The effects of that is only now really hitting home as the children that grew up with that... well, grow up. If it weren't for ageing censorship practices, you'd turn on the TV and all you'd see is 'SEX! OYEAH! *unf-unf-unf-unf*'.
On topic for the question... it seriously depends on the individual. Most will be experimenting with actual relationships around 15-16. Some as young as 13, and some aren't ready for companionship even by 18. It really is impossible to draw a firm general rule - though 15 serves as an average rule-of-thumb as the majority are, or at least are starting to be, much more interested in such things by that age (whether against the set rules or not, teenagers are not well known for following rules if it gets in the way of something they want).
iRamble.
