As long as they are in a safe, draft free, ventilated coop, most birds will probably get into their teens. They tolerate cold better than hot, but I'd like to say my birds are happiest when the weather is between 40 and 60F. As for breed, cornish x do not fit into the average as they are meat birds, and high output production birds like leghorns may be more tumor prone as they are bred for eggs, not longevity.
So want I said can be applied to the average chicken living in the average life style, but really, there are so many variables, it's hard to say. I would guess that the average age of a pet flock is between the 4-7 year old age range. Could be wrong, but that's the impression I get.
Our ten year old white leghorn roo was still going strong when he was killed by a weasel who he fought to protect the flock. His sisters were also killed by predators, but were nine or so when they died and still laying. We have some Golden Comet hens who are still laying but are going gray...don't know how old they are. All ours are free range and live in a constantly changing climate environment.
Bantymum on this site has a 17 year old ( I think she still has her and she is at least that old) Buff Orp hen.