I guess it depends on what you plan on achieving by getting more education. If you want a new career that needs an education there would be a limit. IE you wouldn't want to be over 60 you wouldn't exactly be hirable at that age. But if you just wanted more education or it's a high school equivalency type of thing I'd say any age. I've heard of guys in their eighties learning how to read for the first time.
Normally, I would give a saying I made up, such as "If you're living you're learning." If you want a realistic answer, I can say that going back to school is never a bad thing. If you want to learn more, go back to school. You can always know more.