[Legbreaker]

My motorcycle gear is almost all Kevlar and is over ten years old. In that time I've come off a couple of times and without it wouldn't be here today. My jacket especially copped a hammering in one incident but besides a few small tears which were easily patched up it's still as good as new.
Based on that, and realising it's not necessarily the same composition as ballistic armour, I've no problem in believing it does not degrade over time - I still trust my life to the same jacket, etc that I've been using for over a decade.

This isn't to say that a bullet or shrapnel wouldn't cause it significant damage and several hits wouldn't render it less effective though. General use, wear and tear, or what have you isn't likely, in my opinion, to significantly downgrade performance to the point where it needs to be reflected in game. Any armour in 2000 isn't likely to be all new and shiny though, and is sure to have suffered some damage to it's outer layer from stones, branches and general accidents, but unless it's hit with something powerful enough to penetrate, I say it really shouldn't be effected IC.

Even armour which has been penetrated is better than nothing - a bullet may still hit a good portion of the armour and although the energy may not be spread out across a wide area, it's still likely to slow the bullet to the point where the wearer isn't killed but just injured. 10 penetrations seems a reasonable limit though - after that it's little more than swiss cheese and although weighes the same (near enough) to new offers little measureable protection.