We just went over it with our daughter last year when she was 8. I was going to wait but we were told a story about a little girl in the same grade who got hers and freaked out in school because she had no idea what was going on when it happened!
So I thought about it and turned straight to the internet and looked for a book or something I could give her to talk to her about it. I ended up finding a website that played a little video and talked about it briefly. So I showed it to her and she said "OK, cool." That was it...literally. All the awkwardness I thought was going to be there...wasn't. Now she knows what it is and knows what to expect. No surprises when it happens.
I got mine young and was being raised by my dad when it happened...yeah that was awkward.