I'm 16. I recently lost my virginity from my first boyfriend of 6 months. There is really no age for you to be able to have sex. If you wanted to, you can. But I suggest to lose it to someone who you KNOW will respect you and your desicion about sex and when you are prepared for the consequences it can bring. Just always be protected.
When your hormones kick in a girl starts feeling erotic and enjoys things like masturbation.
That's when puberty happens and a girl, if she happens to find the right guy, could feel the need to have sex as early as 12 or 13. Now whether this is right or wrong is a different topic and I wont go into that for now.
I myself started when I was 13ish and I have come to no harm. In fact it was the most awesome experience and I had a lot of fun.
There is no best age. As I said it is down to the individual and the reasons for having sex vary.