Buying used: how old is too old?

Any of the brands you mention in the 4-5yr realm, even 10-15yr realm will be fine and can be had for decent $$$. As long as they weren’t abused or subject to some other nastiness you’ll be fine.

Bought a pair of MTX Blue Thunders in 1992, 10 inch woofers, still kicking ass all these years later. Purchased a pair of JBL 8 inch woofers at the same time, 10 years ago the JBL surrounds of butyl rubber were shot, they also featured reverse phase, ie. the positives markings were actually negative, a nine volt battery test confirmed the polarity. i now prefer the rolled edge woofers over foam. I have experimented with a variety of woofers from different manufacturers over the years. Cerwin Vega rolled edge, Kicker rubber, and MTX rubber have stood the test of time. I have a pair of EV 12 inch 2 way horn speakers that were born in the late 60’s and still sound good. The vinyl wrap is peeling, but the speakers still work well. Some speakers age well.

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.