How old is too old (for SS gear)?

IME. 15-20 years. Caps start to dry up. They can be replaced if you know what you are doing. Living in fly over country, it is hard to find techs which will work on "stereo" equipment. We live in a throw away society now. most buy les expensive gear which is not cost effective to fix. Those with high end equip are not numerous enough to warrant techs  for our stuff. Specialties are the thing. We ain't special. 😀

30 to 40 years....there are original Sugden class A still going strong even today....

It really depends on your expectations for sound quality. If it is not important, then they will work for decades. But there are deterioration of original sound quality as mentioned above and the advancement in sound quality / unit cost. 
 

if you are trying to put together the best system possible with a mid-fi or higher budget… over ten years is unlikely to be good… unless you get a great deal on something a little over that.

I agree with artemus_5. PS caps often fail after 20 years. My BEL 1001, Bryston 3B and GAS Son/Thoebe preamp all need new caps. The Sumo Andromeda in present use is still working and sounding fine despite being over 35 years old!

Andromeda amps benefit from re-capping, just like the rest do. Like many good designs, they can ’hold on’ and continue to deal with the complexities of audio signal manipulation and the overall control of the electrical pathways involved, but... sooner or later they fail. and just like the rest, they may show little to no outward physical signs of failure.

One big problem is that the slow ponderous and dirty and veiling and crunchy sound that aging capacitors can evoke in vintage gear (Eg, late 70’s old school receivers with no servicing, etc) , is sometimes considered by some, to be the essence of that ’old school vintage sound’. Re-capping properly (correct choices in capacitors) and listening again - can illustrate that clearly.

There is some claim that some older capacitors measure fine and that is true, but the general problem is that leakage, or drying out of capacitors, across time is a real thing and the few that are bad in the given receiver (there are invariably some,and some half gone, etc) can really wreak havoc. The kind of havoc like a main seal blowing on a submarine that is a thousand feet down in the ocean.

(I’ve taken apart ’working’ amps and found things like 35v/100uf caps that are so dried out that they measure 0.1uf, and similar. Yet the amplifier is turning on. functioning correctly? no. sounding a bit ’off’ to the ear? yes. This, or similar, occurs in about 1 out of every 3 ~or worse~ vintage items that show up on the bench.)

Everything is seemingly fine until it explosively -- isn’t. My point is..consider servicing the Andromeda.

You are playing a game that inevitably and incontrovertibly, unstoppably, over time... is shaping itself into high stakes roulette and the seriousness of your game is getting deeper and deeper into the ocean, every day. This game plays out with the house always winning, in the end, unless you switch up and out the given game parameters.

The game plays out with thermal/electrical stressing over time, being the main game parameters.

And that is generally, a thing that only an experienced tech can figure out by casting an experienced eye over the given chassis, boards, and ventilation, thermal loading in the overall design and execution of said unit, parts locations (thermal and electrical load on individual capacitors, etc), and, last but not least..the environment the item/amp/tuner/preamp/etc was used in, in it’s aging/ownership, up to the given service/benching date.