My brother, sister, and I all had braces and the retainer afterwards.
I was 13 when I got mine, had them on for 18months. Not sure how old my brother was when he got his. My sister was like 14-15, i guess.

I didn't really have any problems. I didn't hate them, just dealt with having them knowing that I'd have perfect teeth in the end. They hurt. Lol. I had to have my jaw widened (sp?) with a thing on the roof of my mouth. Maybe they may have been less painful if I didn't need that. no idea.
As for the retainer I wore mine like I was suppose to until it didn't fit anymore. I went back to get it refitted but it still wouldn't fit (I got wisdom teeth or something so it prevented the wire part from going around the back tooth. if you get what I mean). I don't feel like my teeth have really changed now either.

My 12yr old cousin got hers on at the start of dec and she seems to be going ok with them so far.
Good luck with your girl getting her braces on.