I'm not sure how becoming a Grey Warden shortens Loghain's life.
The man is 50y old. 20 years already is all he's got left, lol.
Mid-to-late fifties in Origins, assuming that the Stolen Throne takes place during his early twenties.
That's a pessimistic estimate. 90 years isn't unprecedentedly long.
Theoretically, yes, a man in his shape at 65 could plausibly reach 90 even in medieval times. I think then he'd have to retire and get a GW desk job though. Like First Warden. *spits in disgust*
The immune system starts flagging as you get older as well as when you're young, and Loghain doesn't live like a nobleman anymore. And that's assuming that he isn't simply killed in battle when his reflexes slow down and his stamina gradually disappears, killer shape or no.
Of course it's possible the Joining's effects on one's metabolism or "Grey Warden stamina" could carry him through it, completely rearranging the age process in some unimaginable way. Maybe it protects you from other, 'lesser' diseases and automatically keeps you in peak physical condition until the corruption actually takes you. The speed with which some Warden origins turn from relative civilians into indomitable warriors and Avernus' mastery of his own aging through the Taint are rather suggestive.
Also, Grey Wardens as a rule have better access to healing mages than pretty much anyone but royalty.
Good job pointing out that child mortality is what really dragged the average lifespan down back then, with disease and peasant living conditions not helping. A much-overlooked fact considering how many people fondly belive that we're suddenly developing longevity.