The older a speaker is that much better it will sound, but Capacitors and surrounds will deteriorate in time, if they are good rubber surrounds on the woof's this will not be a problem really, but would still suggest replacing caps for another level of performance, not really because they are necessarily failing, although they do, but newer caps are far more advanced and better material, (with musical results not just scientific results) now available than anytime 10-20 years ago. Beyond that if you do not toast a voice coil or damage them they could last more than a lifetime as long as the glue on the cabinet don't let go, or the wiring does not corrode to some crazy point.
