I started secondary school in 1999. I wasn't massively popular and was a so I can't claim to know what was going on amongst the wider school population, but in our group of friends, three of us didn't even have a boyfriend until 16/17/18, two lost our virginity at 18 in a relationship (mine wasn't exactly stable but I was happy at the time and still think fondly of him) and one at 17, also in a relationship. However lovely this sounds I do remember us all being absolutely desperate to lose our virginity to get it over with and being glad when we'd reached the milestone, and me and my other 18-year-old friend were convinced we were really old to still be virgins
We all used to mess around on over-18 chatrooms (back when Yahoo! had them) as a joke, usually at sleepovers, and would find it hilarious when we got sent cock pictures, and used to take the piss out of the weirdos who were on there, like one guy kept insisting on typing things like "I'm licking you out now and you're moaning, you are sooo wet" etc etc and wasn't paying attention to anything we said, so we started typing random things like "I have a pineapple on my head" etc. We would never have given our real names or ages or where we lived etc, or arranged to meet anyone though. However, the fourth girl in our friendship group was quite naive, had a difficult relationship with her mum and a bit of a turbulent home life, and used to go on the chat rooms herself seriously and met a man on there who was 23 and who ended up being her boyfriend, even though she was 15 at the time. I think they had sex when she was 15 as well, though I can't remember. It was only that at the time we thought this was par for the course and then much later when we were all older and thought back that the rest of us thought wait a second... there was something very very wrong there.
I had another group of friends outside school, one was in a stable relationship from about 15 to 19 and had sex somewhere in that, another is still a virgin as far as I know, and the other was raped when she was 15 :( and then went to residential college away from home at 16 and there was a lot of casual, unprotected sex going on there. I remember an incident where 3 of the girls in her dorm had slept with the same guy and got an STD, another friend had slept with another boy and got a different one, and she was worried she had both. I feel bad now because we were quite judgy about it at the time (because we were worried about her) but now I realise she was trying really hard to normalise what happened to her and "fix" it if that makes sense.
At 13/14 I was completely oblivious, so I can't speak for that, but I get the feeling I was a late developer. I remember being very upset though because when I did start to get interested in boys, around 15/16 or so, I pretty much assumed they'd have already had sex and so that would be expected in a relationship, and I felt sad that I'd missed out on the stage where holding hands or kissing would be acceptable. I wish I'd got the message from my mum as well that some guys wouldn't be after that and it was okay to wait either for one who wasn't or until I was ready myself. She just always told me all boys want is sex, it was meant as a "be suspicious of their motives and protect yourself" thing I suppose, but I took it as "they're all going to want it, so might as well do it anyway or they won't like you". :( If I can do anything for DS and any other children I might have it's to say that even if they're not ready for sex until their 20s that's okay and it's not "weird" to be a virgin longer than your friends. I don't know whether this was just because there was a lot of emphasis on "We know you'll want to have sex, so here's a condom lesson" etc or whether everyone else really was interested though. I seem to remember things referring to how you didn't have to have sex until you were ready, but nothing to say that you might not actually be ready by 18 or so. I just thought that everybody over 18 was having sex and I was broken, or something.
Sorry for the essay I expect it's always been this way - some teens will be having sex younger. 13 wasn't considered that normal when I was at school. 14 was probably okay, but considered early, but most people seemed to lose their virginity at 15-nearly-16, or 16/17, or just kept quiet about it. Likely there were lots who were older and we just didn't ask. I think the internet does make things a lot more accessible though, and with things like facebook it's worrying, when we used to make naked pictures etc they were just emailed!