As I recall...the oldest recorded ball python in captivity was 43 years...and it had come into captivity as an adult, so it's exact age beyond that is unknown.
I've heard of several different people with ball pythons 20+ years old still going strong. But few people have been keeping them in large numbers for long enough to have a better idea of their true potential lifespan in captivity.
Really, a lot of it depends on the skill and dedication of the keeper, I believe.