Speaker life expectancy

I have a couple pair of old Vandersteens. 2c model which I bought new almost 18 years ago and a pair of 2c(e or i) which are about 8 years old bought used. Both sets still sound fine to me, and mine have always been given TLC. There must come a point when speakers go bad. Will this be readily noticable, or have they been gradually detiorating and my listening has adjusted accordingly. Vandersteen has a set up where I can send the speakers back for a refurb (not sure of the cost though). These are used in my HT and I have an audio set up which sounds much better using NHT 2.9s. I always thought the sound difference was due to the different speakers. Now, I'm not so sure. Thanks.