Buying used: how old is too old?

Highly subjective. Totally depends on the design, and selection of parts and especially the quality of manufacture.
Lest we not forget that longevity also depends upon the quality of care and maintenance. Sometimes, people are the worst enemy of how long something man made will last. 
I say these things because I have always been a careful and concerned caretaker of all things I purchase. And most especially my audio gear.
The exception, of course, pertains to audio equipment that contains predictable parts that will eventually reach end of life and will need to be replaced. The example being, vacuum tube replacement. I for one can live with that.

I have 75 year old drivers that will be here 75 years from now and work the exact same way they did when they were first made.

I have hard wood furniture that is 300 years old and silks that are well over 100 yeas old.

Crossover parts that’s about all that will wear out.. Plastic will even hold up 50 plus years in a controlled environment, Bake-o-lite 300 years?

Ribbon speakers, as long as you can fold tinfoil you can fix them..

I have terminal blocks well over 100 years old..

So what’s gonna wear out? Round speakers? Now you see the problem..

They will last 60 plus. I have Watkins drivers that have NEVER been touched and the BR surrounds are still intact. Infinity Infinitesimals.
they will last another 25 years with care.. 7-80 years.

I chipped a front tooth 45 years ago. If I would have fixed it then I wouldn’t have it now. I fixed it with epoxy and my dentist added the final touchup.. 45 years ago.. As a matter of fact it's the only ONE that hasn't been messed with.

100% agree with you clearthinker

I listened to the Tekton Moabs at the Capital Audiofest… in a word - JUNK!

and for the record, I run 40 year old MAGNEPAN Tympani 1Ds that will kick the crap out of most high end production speakers of today (even current Maggie’s).