I have a guy here at my school that is 75 now, and did not even start training until he was over 70. He's also in astounding shape, and even still does competition! Bob is a 4 stripe blue belt now.
Though I recently heard an academy on eastern Long Island signed up a 91 year old man! I would say he's probably the oldest to start training.
40's HAHAHAHAHA! The oldest guy I train with is 65 soon to be 66. I have never tapped him (I'm 34 and not a bad grappler at all). He has been wrestling since he was in grade school and has never stopped. He has FREAKY core strength and a chip on his shoulder. He truley enjoys inflicting pain.
Two old geezers in their late 50's early 60's both were old school judo bad asses before they switched to BJJ. Both are brown belts in BJJ and both have the grip of 600 lbs bears.
And you do not want to meet either of them on a dark street corner 'cause they are straight up old school tough guys.