Life expectancy for speakers?

Can anyone tell me the rule of thumb for life expectancy on speakers? When should drivers be replaced? Under normal operating conditions with "average" use, should one expect 10, 20, 30 years? Lastly, what drivers tend to wear out first, tweeters, midrange, or woofers?

My Infinity pair, purchased in 1984 or 1985, are still fully functional... though the cabinets are the worse for wear. I just purchased a pair of Maggies that were made at least 18 years ago, and they play beautifully.
The only thing that will fail is the woofer surround if it is made of foam. Rubber surrounds will eventually fail and just need to be replaced. (On good speakers, the whole cone can be replaced)
I would say the lifetime of a well cared for set of speakers is 50 years. (you can still buy functional speakers made in the 50's...)
If you don't over-drive or under power the speakers, everything except the woofer surrounds should last a long time. My local speaker manufacturer/rebuilder recently told me the much advertised Sharper Image washable (no filter) air cleaning machine generates ozone. He says this will appreciably degrade foam woofer surrounds.
I wonder how long before this becomes common knowledge?