I guess there's no standard age for that, when my younger brother was 12 he was making out with a girl in the movies and they were deeply in love texting each other every day about how they were 'the best thing that ever happened to me'. And it looked ridiculous to me, especially when I remembered myself at 12.
In any case, you can't forbid her to do it. I mean, you can, but criminalizing this sexual behaviour won't do any of you any good. And it won't prevent her from doing it anyway. Make the best of the trust that you seem to have right now, and start a sensible 'sex-ad' campaign. Find out where she's coming from - are her girlfriends also 'dating' ? And classmates? I remember at this age at school we used to have a lot of sex talk and crazy rumours (or maybe not that crazy..) so if you're able to discuss such things with her it can help her develop some correct values and opinions about appropriate sexual behaviour.