As others have already said, you are never too old. However I would add that you have to make sure that you examine your expectations closely and ensure that they are realistic. I have taught students who started in their mid 50s, and even one in his mid 60s. Obviously these guys train as hard as they can, but they also understand that they are going to be limited in some things - like full contact sparring.

Jump in and try it out, and do as much as you feel you can do.