What's the best age for kids to start training in MMA / UFC?

Like it's been said, there is no set age that works for everyone. I'd say you can start getting them into the core discipline such as wrestling, judo, boxing, BJJ, karate, taekwondo or whatever probably about age five. It's a great way to learn things like respect, work ethic, teamwork and so on. I don't know of too many places that actually have children's MMA programs, but again I think if you can find one, five is probably the earliest I'd want them to start training.