SPEAKERS: Can they / do they wear out?

I reckon that based on the serial numbers of my Briks they are late 70's/early 80's. Looking at the B139's the rubber diaphram is a pale grey colour rather than black and it feels fairly firm to the touch - almost 'dry' if that makes sense? Now, I'm wondering if speakers degrade with age and if anything can be done to help prolong the ,,,,