I think it's worth remembering that the low average life expectation of pre-industrial societies was mostly caused by all the infant mortality going on. People weren't all dying at 40, it's just that a lot of their children died before adulthood.
I think it's also worth noting that a lot of pre-war knowledge would be remembered, putting even tribals (possibly) ahead of say, the medievals, in many aspects. Things like "clean your hands with water and soap" weren't universal among the medievals, but it would likely remain in the wasteland.
I think average lifespan is like 40-50. Probably pushed up by the odd technotopia but also pushed down by all the tribals and shit towns.
It would depend on a lot where you live. Somewhere in the NCR is probably not that far from that of your average western country, somewhere like 50-70 - through it would depend on wherever you are in the "Core Five" states in well-off places or in the frontier boonies. If you are rich, you can probably pay health treatment from a group like the Followers, or go to Vault City.
I think the longest life expectation is probably somewhere like the Brotherhood of Steel. Between their technology and their quality >>> quantity approach to things, I suspect the Brotherhood might have comparable lifespans to a modern first-world country, perhaps even more - modern first-world countries don't have access to auto-docs, stimpks and cybernetic implants. Enclave too, for similar reasons.
The Institute is another worthy contestant, and would have likely broken all records if they weren't idiots who shut down their own cybernetics program even through its obvious it could have led to extended lifespans, perhaps even practical immortality. With their tech, they could probably print and close organs easily - if you can make a synth you can print an organ.