How long should speakers last?

I've had a set of NHT 2.5i speakers for 18 years. They still play great. How long should speakers last? Is there a rule of thumb?

I am about to purchase a similar age set of mirage m3si. Assuming they were well taken care of, how long would you expect they last?

Caps and foam surround can go but are serviceable. Therefore, speaker can last forever.
For what it's worth, my 1997 Mirage M190is pair still does daily service in my office system. Butyl surrounds last quite a while.
I have 1996 Aerial 10T, but I replaced blown woofers on both due to the cranking habit:-)
A good speaker should last 45 yrs,Happy birthday.
My kid uses my old pair of KEFs. Still sound good to me. I am sure the caps could be replaced, but plugged into her Yamaha receiver it still makes good music.
Like you Sm, I owned a pair of Infinity Reference 60 speakers for 18 years before I sold them. They never skipped a beat in that time & still looked mint when I sold them. Though the original foam bass surrounds on the two 8" IMG woofers were drying out and becoming deformed (which is to be expected), resulting in distortion. I would have also expected the caps to have degraded & the internal Monster cable to have become somewhat oxygenated.

In saying the above, it's all relative. The Ref 60's were only $2500AUD way back when, and obviously capacitors, inductors, OFC wire and speaker surrounds have all improved significantly since then. Therefore I would expect a pair of good hifi, or high end speakers to last >18 years these days. How long would vary from speaker to speaker, though if you're paying big $$ for high end speakers you should rightly expect to expire before your speakers do!
No easy answer to this question. Some speakers used material for the surround that deteriorated after a fairly short time and would turn to dust. This was primarily a problem with speakers made in the 1980s (I hope no one is still using that kind of material).

I have a set of horn compression drivers that were made around 1939 that work perfectly and are still very highly sought after.
It seems like dynamic drivers can last if the surround is made of Butyl (rubber) rather than foam. I have a pair of early-70's ESS Transtatics with KEF drivers (B139 and B110) which are still fine.
Treated cloth surround equipped bass/mid drivers are known to last decades (under proper use and conditions of course, which goes for all kinds of drivers), and when "hidden" in horns can look mint when dug out even after 30+ years. Same goes with domestically used compression drivers, many of which (also per Larryi's example above) appears to last virtually lifetimes. Less talked about perhaps is the cabinetry, the maintained aesthetics of which I gather can vary a lot. Solid wood cabinets (though too rare) look gorgeous, but the panels/lumber need a proper drying period for them not to "work" after assembly - critical particularly when used as horns. Moreover, proper veneering, where such is used, is not always a given. Cross-over components (i.e.: capacitors) seem to have improved in regards to their spec longevity since "earlier times," and where quality items are used should last decades, or so I presume.