Glass doesn't really 'decay' per-se.

The following issues are common failures in lenses:

• Scratched glass

• Fungus/Mold on lens elements

• Damaged moving parts/mechanics (autofocus motor, aperture controls etc)

All of the above (with the exception of perhaps the fungus issues) happen on lenses as time goes on and they are used. If you were to store them in the correct conditions for a long period of time i'd expect lenses to last almost indefinitely.

The reason why people won't use old lenses now on newer bodies is that they just aren't compatible, and this is what really 'kills' lenses. They die because manufacturers make them obsolete and as they break people don't bother getting them repaired because its cheaper to upgrade.

Naturally, the quality of the lens you buy will also affect your purchase (for example a full metal, completely sealed lens will presumably last longer than a cheap plastic kit lens.)

There are plenty of examples online of lenses that are DECADES old that have been well worn but still looked after and are still working perfectly like new.

Lenses, like all mechanical precision devices need to be maintained, make sure you do this and have it done professionally if you can.

You should also check this related thread here