Deteriorating speakers -- Class A today, C Tomorr

All speaker drivers, as well as the internal wiring and crossover parts need at least 100 hours of usage before they reach 90-99% performance level. Especially the driver suspensions need to be loosened up in order to move in and out rapidly and uniformly. I've owned over 20 pair of new speakers in the past 15 years, designed and built even more, and all have sounded worse then a broken in set of demo speakers, and did not have the proper low frequency weight or high frequency speed, delicacy or extension until about 100 hours or more. Also, the wiring, resistors, and film capacitors need to be "burned-in". As far as the effects of old age on drivers, as long as the surround around the edge of the cone is not foam, (deteriorates rapidly), cloth or butyl rubber will last and remain consistent in performance indefinitely. The spiders surrounding the motor/voice coil also tend to hold up indefinitely in my experience, however, they can wear out with heavy usage and allow the voice coils to be off-center, which is rare with high end audio gear/usuage.
Also, with use the driver and other mounting screws can become loosened and require tightening from time to time, which can also improve performance.

As dombubble above, I've found that speakers can last a very long time. As Oz notes, some parts may need replacing, but generally they perform. Technology, however, doesn't seem to last that long, and materials & parts improve -- or at least, become cheaper & thereby what was found only in giga$ models yesterday can be had for just(!) mega$ today. In that sense, "overpowering time" is a blessing.
(But the Grande Utopia are mega performers, today too, IMO.) Maybe we shouldn't worry because as W says, we usually change them before they kick the bucket... Cheers

I have gone and auditioned some pretty high end gear and compared it with my system at home, the Infinity IRS V for my mains and I have some very low serial number betas as well. They still sound class A to me.

Humphry - All of the above posts have mentioned some very valid points to consider. Personally, I've probably had only 7 or 8 sets of speakers over the past ten years; and can't honestly think of any problem I've ever had with any used speakers I've bought. In fact my Thiel 3.5s, which I bought from the original owner, are about 10 or 11 years old and sound better than ever. (Partly due to better sources and electronics over the past few years!)
Mostly what I would be concerned with is the past user's treatment of the speaker. Does he/she play music at high levels? Listen to lots of rock/rap/heavy metal? Do the speakers look like they've had a rough life? (Probably a good reason NOT to buy them!) Let's face it - in this hobby of ours - condition counts for a lot! Do your homework, and by all means if you're going to make a speaker purchase listen to them BEFORE you buy. Just because a speaker is highly rated by TAS or Stereophile doesn't necessarily mean that it will sound great with your equipment. Good listening!