pre-crisis Superman's lifespan

Was he immortal?

no he was born on kryton, hes an alien not a god.

I'd like to know if post crisis supes was around when the JSA were (in post crisis continuity) or if he only started heroing after them

I think the general consensus is that as long as earth's yellow sun exists, so would Superman.

I dont think superman will live to be a billion.

Post crisis supes actually outlives earths normal yellow sun

Originally posted by eleveninches
I'd like to know if post crisis supes was around when the JSA were (in post crisis continuity) or if he only started heroing after them

He started after them.
In fact, he often says the heroes of the JSA inspired him to don the cape and tights.