22 is a great age to start. I was a wrestler for all my life, but didn't actually pick up any form of Mixed Martial Arts until after I left high school.
I got an interesting question during a Brazilian Jiu Jitsu class the other day, and I truly did not know how to answer it. The question was “What would be different about MMA if the bout happened in zero-gravity?
I put a pretty heavy emphasis on using the Martial Arts that have proven effective in the UFC. In my opinion, all martial arts have a place in the world, but because I fight professionally, I feel I need to be versed in the hand-to-hand combat that has proven most effective amongst professionals.
If your goal is to simply burn 1000 calories an hour while learning a new skill that could potentially save your life, than take a cardio kickboxing class. Every city has gyms that have Cardio Kickboxing classes, I would find a gym that trains pro MMA fighters, they tend to be the best.