I first tried when I was 17, and a few times after that for a couple of years. I got too frustrated and couldn't make it work. Finally I started using it regularly around 20 after "other" things had been up there ;)
sorry this is posting under my husband's name. my daughter is three and a half, still potty training, so she is in the bathroom with me a lot and sees me using the cup. she knows all about it and how it works, it's kind of funny. I think since she has been exposed to it, it will be a lot easier for her to use it herself when she gets old enough. I would definitely offer it to her. Or cloth pads if she would rather. Or both.
No harm to introduce it to a younger girl, but I think she'd only be able to use it after being comfortable with tampons. It would seem too large and intimidating for a 12 y/o in my opinion. But make it available for when she's ready.
My daughter is very young still, but I'm planning on getting her one when it's time. I'd imagine young girls would have trouble with it in the beginning though... I myself didn't even feel comfortable using tampons until I was 17. Maybe with some good education though...
A teenage girl might not have the patience to get the hang of this, but it's worth at least trying. But, girls will probably struggle to get it in with an intact hymen...so the insert that comes with the cup says.