Personally, i say whenever they want to. if they are 16 then 16, if they are 13, then 13. whenever they feel like it.
There is no magic age or number that can be applied across the board, some people will be ready for that kind of thing sooner than others. by the time i was 12 i was running round trying to finger girls and all sorts. I was far younger when i first got my hands on some dirty magazines. kids will learn in their own time.
I would imagine, if they are anything like me that they would only do it once anyway. before deciding that it is crap. i would have been about 11 or 12 when i first called one of these sex lines you see advertised on tv and in news papers. it only took 1 call for me to realize that it was crap, and that all it was designed to do was extract money from people, and have never called one since. i imagine many kids would be the same.
they will either try it once and declare it to be shit and pointless, or it will happen more frequently, instigated by boys, lead on by girls who think its funny. as for kids and teens meeting up and having sex because of it, i think the cases will be few. as most girls of most ages will back out of doing anything at that age.
note: the age of consent here is 16, and i think its a good general number. 18 is too high, 21 ridiculous.