I would agree that 12 is too young for a marathon. My oldest son ran Chevron Houston "with" me last January. He was 17 and a senior in high school. My younger son is much more competitive and I know I'll have a tough time holding him off until his senior year.
My concern with them training for a marathon at too young of an age is the stress on their bodies if they are still growing quite a bit.
I personally think that the experience with delayed gratification that one gains from training for months before experiencing the joys of a marathon finish is worth every hour they spend training. It's something most kids don't get in the instant gratification world of today. You have to give up some things when you're a senior in high school and you've got an 18 mile run scheduled at 5am on Saturday morning, but you also get to experience something that only a very small percentage of the population ever achieves.