Most tubes spend the vast majority of their life changing very little, and at the end, they decline quite rapidly. But even then, when they begin to test weak, depending on the amp design, they can still sound fine. I had a set of 6sn7s that began their end of life plunge, as measured by my Amplitrex tester, so I contacted the manufacturer to see if I could safely run them. I was told that the amp should sound fine until the tube is very close to death so I should run them until they quit. In my current setup, i run tubes of unknown usage prior to being used in my amp and linestage and so far, they are still going strong for the past 15 years (348’s, 349’s, 310’s).
Caps, particularly electrolytics, do age; my only point is that they typically don’t do so as quickly as some think. Some resistors, particularly carbon composition types, will in time drift in value. But, to me they sound better than other types in certain applications so one lives with that. I know a builder who uses some extremely old resistors in his best builds and his stuff is crazy good. There is nothing as pretty as seeing sixty-year old IRC resistors used in a new amp or restoration.

