heh, i keep asking myself the same questions :P i'm 22 as well and i almost kind of started to train (but not anymore unfortuneatly). i'm not looking to get into shape or something, i would just like to one day get in to a cage with other guy and find out what i'm really made of.
it all depends on capabilities and dedication. If someone is extremely eazy learner, have plenty of free time and a bit of money, starting mma at 22 years old he can become a proper fighter in few years (with a bit of luck).
I hope we will be able to achieve our goals mate, good luck