I started "dating" at 13... of course, dating at that age was getting your parents to take you to the roller rink or movies and if you held hands you were tha supah pimpx0r.
Let them dating when they want to. It'll progress slowly but they need to make those first few years of just dealing with the opposite sex. They aren't going to go out and bang the football team while they're 13, but they still need to learn about dating in general. If you forbid them now, they'll lash out and do more than they should be to rebel. Just make sure you do your job as a parent and give them all the talks and support they need. And keep track of their life. Know who she's hanging out with, what they think about the other people they hang around with, basically be a part of their life that they can come and talk to. If you're doing your job as a parent, they'll come to you for advice and listen to you.