Assuming they're in Brilliant Uncirculated condition, you did the perfect thing already !!
The first year of these was 2007, and I'd suggest that you try to get one of each President before too many years pass. That way, when she's ready to receive your gift, she'll have a complete collection of them up to that point.
As far as the age......
My 6 year old loves coins and collecting......but not to the point where it's a "realized hobby" yet with very much focus. I would guess around 10 years old or so this might be more prevalent, as long as I keep her on it.