My 10 year old has been flying for a few years, and is now at the point where she's building fiberglass mid power rockets on her own (With my supervision). 12 is definitely fine, although you'll probably want to help them build and definitely help them launch.
I'd suggest getting them a small kit like an Estes, and build that together. Then find a local launch, take the Estes out there to launch with them, and let them also see the larger rockets that people will be launching. At that point, they'll probably either like it or not. If they do like it, talk to some of the people at the launch and see about joining the club for future launches. If they don't like it, you're out the cost of an Estes kit.
Good luck, and have fun. This is definitely a fun family hobby.
Here's video of her rocket launching. We launched it with a high power motor (Aerotech I205), so technically it was launched under my name, although she built almost the entire thing herself. According to the simulations, it went well over 1,000mph and up to several thousand feet. She was absolutely over the moon watching it launch, and even happier when we successfully recovered it.
Video of the launch that she took, you can hear how excited she is.
[video=youtube;s08_FB1Y3_c]https://www.youtube.com/watch?v=s08_FB1Y3_c[/video]