Buying speakers ten or more years old a good idea?

It's like anything else. Some products, whether speakers, amps, or cars, are
classics distinguished by a synergistic combination of forward thinking, sound
engineering, meticulous manufacturing, and high performance.

One sweet spot is the $400-600. For a new speaker that's a decent pair of
stand-mounted monitors with good midrange, but limited sensitivity, bass
extension, and dynamic range. For the same money you can pick up any number
of lightly used room-filling floorstanders such as the Aperion 633T or Mirage
M5si with transparent midrange, smooth treble, and bass that reaches down to
the low 30s or high 20s. That'll also save you the cost of augmenting the
monitors with subwoofers, which is an inevitable add-on for some.

Also, some used speakers hold up better than others depending on parts and
cabinet quality. Speaker cones with foam surrounds will need the surrounds
replaced about every 20 years. Those with butyl surrounds (e.g., ADS) neverl will.
Some older speakers were heavy and well-braced, others were not and have
more cabinet resonances. Examples: The Aperions I mentioned use 1" thick HDF
for the cabinet and they weigh 77 lbs each. My Mirage M5si's also have thick
walls, are well braced, and weigh 85 lbs. each. ADS speakers are sealed and well-
put together and also have very inert cabinets along with the butyl surround
woofers.

The L-R speakers that anchor my HT are Mirage M5si's, bought 17 years ago. I have no intention or reason to replace them. Ivan_nosnibor's post has some good advice I need to follow: redo the crossovers and top off the ferrofluid in the tweeters.

Crossovers: A few years ago I was using a 5-channel Adcom to biamp the Mirages and to power the center channel. I kept blowing a fuse, and an ohmmeter indicated that something was wonky with the individual halves of the crossover, while the full crossover presented a reasonably stable load. I went back to bi-wiring, but the blown fuses do point to a crossover problem.

Tweeters: When I run my pre/pro's automated speaker setup, the white noise indicates some differences among the four tweeters (they're bipolar) in the M5si's. While that may also point to the crossover problem, it may also be the ferrofluid (assuming my tweeters use it). So anyway, I am overdue to take those in for some maintenance, after which I will REALLY have no reason to replace them.