Martial arts, from experience, isn't always best to be learnt from a young age. This may not be true in tae kwondo, or MMA, but from my experience in both judo and karate, the kids are taught very basic stuff, even at the higher grades. For example, at my club, the equivalent of a blue belt (8th belt) for a child is the yellow belt (3rd belt) for an adult. As I said, this may not be true in all martial arts, but I know that attaining a black belt as a child at my club is a rather simple affair, and it shouldn't be, hence why the difficulty is stepped up greatly for adults.
As for which martial art you should choose? I have no idea, although I've heard MMA is the proper, dirty fighting stuff.
