Assuming no abuse over the years (played too loudly or with a clipping amp, etc.), the things that deteriorate most with age in a speaker are:
- driver surrounds. This is most pronounced with foam surrounds. If damaged, it will be very obvious. Other surrounds may still be fine -- I have a set of KEF B-110 mid-bass drivers from 1979 and the surrounds are just fine.
- electrolytic capacitors in the crossover.
- ferro fluid in tweeters can dry out. Many tweeters do not use this fluid. Check if this applies to the tweeters in your speakers.
- driver surrounds. This is most pronounced with foam surrounds. If damaged, it will be very obvious. Other surrounds may still be fine -- I have a set of KEF B-110 mid-bass drivers from 1979 and the surrounds are just fine.
- electrolytic capacitors in the crossover.
- ferro fluid in tweeters can dry out. Many tweeters do not use this fluid. Check if this applies to the tweeters in your speakers.