Speaker Life

If it is a high end speaker probably longer than you live yourself - given the fact of proper use in ANY respect (no overloads whatsoever, no direct sunlight, no bumps whatsoever, no clipping amp etc.)

A speaker from the 80's will suffer from foam surround rot. This is deteriation of the foam 'ring' which connects the cone to the frame. Second item of concern would be the caps. Electrolitic caps may loose storage capability or take a memory set or even leak over time. I assume you are talking of decades here and not years.

Yes I am talking many years. My current speakers are 25 years old (have changed everything else more than once) and I am concerned about transistors and capacitors in the crossovers beginning to operate outside of specs.
I am now looking at upgrading with used speakers and wondering about the wisdom of getting very expensive when they were first bought and now 10 plus years old or more, and therefore now cheap. Comparing this to moderately expensive and say now 2 years old such as Sophia's Watt Puppies etc.

Speakers that don't use foam surrounds don't suffer from foam rot.

I don't think you can generalize....a lot depends on the speaker manufacturer and their design philosophy. It can probably vary from as little as 5 years to much longer.

At one extreme are pro studios, where robustness is very important, usage is high, down time is a nuisance and consistency in response is expected over many years. For example, ATC provide robust speakers for the pro market and claim an upgrade path going back over 25 years. All of which is helpful in the case that something breaks or wears out;

http://www.atc.gb.net/news/2005/FrankWolf.html

This makes me tempted to generalize that it will be safer buying older gear built for professional studio use than older gear built for consumer use. However, even companies supplying studios can go out of business and shut down manufacturing!

Perhaps the safest advice in second hand older speakers is to stick with a popular model with many years of production, a good track record, and built from drivers/tweeters that are still widely used. At least there will be parts available!

My Jensen and EV speakers are from the late 50s early 60s. Paper surround still in excellent shape. I gues the can last over 50 years with proper storage.