Yeah I think that what PS Audio said to you is a little misleading.
It would depend on the quality of the product. But I even have a couple Dynaco preamps that I can personally date back to the 70's that I can plug in & they work. I'm sure some parts are out of spec though.
I have an Assemblage preamp which is on offshoot from Sonic Frontiers built in like '98. It was modded to the gills with top of the line resistors & overkill pricey Hexfred diodes..the power supply electrolytics were replaced with Films in the original build. I still use it occasionally & it sounds nice. I do get an audible scratch from the Nobles volume pot when I pass the 12 o'clock position.
So I think it depends on the product. 20 years is probably time for some electrolytic replacements but many old tube products are still alive & kicking. Well I did have a tube preamp manufactured in 2012 with a very small chassis that I had to replace the electrolytics after 11 years. Couple bulging caps but just replaced them all.