While as explained the vest never really expires, it just degrades with the moisture. More importantly is the fact technology changes as does certification procedures. The vest get safer and lighter, and wearing something old is screwing yourself out of your own safety.
How much is your own life worth?
Is it worth $6 every two weeks? That is about $0.50 a shift if you work ten days a pay period. If you take $6 every two weeks when you get paid and drop it in a jar, after 5 years, you are at $780. A price that can get you a custom fitted, top end, light weight IIIA vest from a major manufacturer.
If you agency chips in, it is even less money. And quite frankly, DOJ has a an idiot proof 3 step system to get vests 50% paid for. Really is no excuse for not having one every five years!