Larry Gibson, I agree with your statement regarding "weakening due to age" of steel. I would add that failure of steel due to stress occurs BEFORE catastrophic failure. The controlling facture is YIELD strength. Up to a given alloy's yield strength the steel is elastic under stress and will resume it pre-stress state once stress is removed. Once yield strength is exceeded the steel will not withstand the same level of stress as it originally did.
The caveat with old milsurp rifle is that we do not know what has been done to it before we get it. That said, any properly designed firearm is designed so the strength exceeds the required strength by a safety margin.