Years ago, IACP and NIJ came up with a "rational replacement policy," recommending replacement every five years. Moisture, UV exposure, and other factors eventually degrade the vest fibers and reduce ballistic protection. The Bureau of Justice Assistance adopted the five-year replacement schedule for their vest grant program, also. Original Kevlar, Kevlar 129, and Twaron probably are fine well beyond 5 years, but the Zylon debacle a few years ago showed that Zylon degrades much more rapidly than ever expected (or at least than ever publicly admitted!). The most famous case involved the failure of two "Second Chance" vests, but it really had little to do with Second Chance. They just had a huge share of the market, and so it was their vests that showed up first when the problem was discovered. (I currently wear a Second Chance Level II vest in 100% Kevlar 129, and have all the confidence in the world in it. For those of you as old as me, my very first vest was a Second Chance "Model Y," which even predates the NIJ standards!) There is a new fabris that also had a shorter effective life than expected... Dyneema, or something like that? NIJ has very recently published their new body armor standards, and will begin certifying vests (again) in the near future. (They have been operating with interim standards for 5-6 years.)