My Mum did this for my sister when she was about 13, so I was 11 or so. I used to read them too (we shared a room)
(And she didn't actually do the subscription, she just gave her a card that said your magazine of choice once a month for a year- could be a way to get around the gift issue, although is a bit more expensive)
Honestly- reading all that stuff about periods, sex, dating etc helped me a LOT. Even now I'm the one my friends come to for advice about all the same stuff (Not saying I am an expert at all, just that Dolly gave me a kick start in that area!)
I think it's a great idea.