In terms of actual penetrative sex I was a later starter than thir, but I had been playing sexual games of stripping and genital manipulation (usually also involving bondage and play-torture, but that's another thread) with siblings and friends from about age 11. I have no evidence that this is unusual.

The fact that none of us considered including penile penetration in our games might have been influenced by the fact that we had been told what it involved, and could share an unspoken concensus that it was too serious for games, just as we wouldn't include a real knife in play-fights.

But from age 13 I was certainly ready to fantasise about it, and if I'd been lucky enough to have found a like minded partner I'd have been keen to try it for real. Again, I've no
reason to think this is unusual.