The games are pretty fun, so she'd probably enjoy the Balance and Aerobics parts. Aerobics includes hula-hoop and super hula-hoop (once you've unlocked it), jogging (running for a few minutes in place), step and a few others I haven't unlocked yet.
Balance has skiing, ski jump, snowboarding, a game where your Mii is dressed in a penguin costume and has to try and catch fish, a game where your Mii is enclosed in a bubble and has to float down a river without touching the sides and a few others.
The better you're able to control your balance and center of gravity, the better you do. There are guides in the upper right corner that shows you where your COG is and where it should be for that particular game.
The Yoga and Muscle sections use a trainer, no Mii, and graph your balance while you perform different tasks. It would really depend upon the child whether they would like it or not. I can say that DH has been doing the yoga exercises, though I have never been able to get him the least bit interested in yoga before. It's a small victory, but victory none the less.
I would think it could have the same result with children/teens -- because it's done using the balance board, and there's the challenge of keeping your balance within a certain range, it could expose them to something they might otherwise be unwilling to try.