Okay, there's no right or wrong in this. Physically girls differ (I presume we'll stick to women here!), you are physically ready when you get your period, your period means that you are fertile, so therefore can have a child. Mentally it's a different thing though, as you've already said. However, in saying that you are physically ready from when you have your period is accurate, but studies have shown that the earlier a girl has sex the higher her chances are for getting cervical cancer at a later age, therefore it is understandable that people think you should wait.
I would say around 16/17 is the normal age that girls should start to have sex, you are mentally more ready for it and physically you are basically full grown, but everyone has their own opinions on this. Personally, I think you should do it when you feel ready, that is that it is something you want and not because your boyfriend or friends are saying that you should. Peer pressure is a bummer in this situation!
French kissing used to be around 13, I think.. I know it was for me! I'm afraid I don't know what the age is around now, but it wouldn't surprise me if they were starting younger!

Tash