Buying used: how old is too old?

Nope 5 years can even be a current model. Like Revel Ultima for instance. I say 20 year is fine if the money is right. I’ve had 30 year old speakers and they sounded really nice and were original to my knowledge. 

I buy all my gear used and the only negative surprise I ever had was not from the purchase but from Technic and the parent company Panasonic. I found out these companies could give a rats patoot about their customers. A word to the wise never buy from either of these companies regardless how good the product is. The product I got was excellent works fine but if you had a problem good luck trying to get help. I just wanted basic info. like headroom, or does the amp have limiter, what the signal to noise ratio useful information the manual did not have. their phone numbers don’t work or you get a recording sending you to a phone number that does not work one recording got close saying there is a 40 min wait time and if the product was over a year old they were going to charge you some mumbled amount on the barely coherent recording, F them.
I’ve dealt with PS Audio, Emotiva immediate and friendly help was a phone call away.

Bought late model mint shape infinity IRS Beta’s and vintage very high end powerful amplification. Speakers have changed but insanely good from the past Is still fantastic even by today’s Standard. All the fairy dust and novelties will not take modern low end and make it anything but that. A lot of BS out there, ask PS Audio to put Tekton Moabs in their sound room…. Delusional 

I have a pair of Polk Model 10's from 1979. I remember the day I bought them. They haven't, to my knowledge, degraded yet. Maybe if I could A/B them with a pair of new Polk Model 10's (impossible), I would hear a difference. The world will never know.

The driver manufacturer Accuton says their speakers are designed and engineered for a lifespan long enough to be enjoyed by your grandchildren.