It won't be true that "all his friends" are allowed out til that time, but it's probably true that some are - or do, even if they're not 'allowed'. My DS1 (now 18) definitely wasn't allowed to stay out that late, but I suspect he sometimes did. After a while, I cottoned on to the fact that some of the arranged, agreed 'sleepovers' (i.e. "Can I stay at X's house on Saturday please mum?") were in fact used as opportunities to stay up and out very late.
I found 15 the worst and most difficult age for this: he still felt so young and immature to me, but he obviously felt very grown-up and mature to himself. And at some point that year, he realised I could no longer stop him from doing things he wanted to do: if I grounded him, he went anyway. And he wanted to do things like this SO badly that he didn't much care about subsequent punishment.
One of the things I find myself saying quite often is that around this age, parents of teenagers sometimes feel they're 'losing control' because their DC is doing things that parents dislike or have banned. It's a horrible feeling; when it happened to me, it made me sad, panicky and angry. However, with hindsight, now my DS seems to have grown up and become much more pleasant again, I can see that even when I lost control, I still had influence. Ultimately, I think you just have to do what you think is right, and keep on giving them moral messages'... and even if they appear to reject these, they do seem to absorb them in the end. :)