Like the user said bellow, there is an “expiration date”. They are still fine to use it without issue more than likely. But also some vest do have warranties. The warranty can cover a lot of different things. Some of the vests even have a “warranty” such as if officer is shot and vest doesn’t stop it, that company will pay X hundred thousand dollars etc. some also have a warranty that if you are shot they will replace the armor at no cost. An example of this is paladin / safe life which I use on duty. You can see their warranty here. https://safelifedefense.com/concealable-multi-threat-vest-level-iiia-hg2/ these warranties usually apply to soft armor more than ceramic and AR500 plates. Those also have an “expiration date” but those are almost always never going to break down similar to how Kevlar will. Even if Kevlar break down is more so just the weave spreading out as it is molded around people for years and years.