Amplifier's age

I see posts about an amplifier's age and how it's getting long in the tooth. What does that mean? If you maintain your equipment, caps etc., it's well made to begin with, it doesn't use rare transistors and such what difference does it make? Maybe a new amp sounds better, maybe it doesn't. I see amps especially SS McIntosh amps that were made back at the dawn of transistor technology still plugging along. Some tube amps are 50-60 years old. Is it because we always have to find the next thing?