Airframe life is rated in hours of flight, not in years. So your 50 year old Cessna might still have plenty of hours left in it; I bet planes like that aren't in the air as much as commercial airliners.
Also note that the GP pointed out that it depends on what the plane is made out of, so older construction from 50 years ago can have more longevity in a way that more recently-built planes will not.
I responded to someone who said it is not safe to fly airplanes for decades. I don't think they meant "decades equivalent of flight hours", but the decades since manufacturing date.
Also note that the GP pointed out that it depends on what the plane is made out of, so older construction from 50 years ago can have more longevity in a way that more recently-built planes will not.