TexasCHLforum.com

While I have no idea if this is true, I have read that the fibers inside a bullet-resistant garment can degrade and separate due to movement. For example, if an officer is wearing the vest all day, the vest is rubbing against the car seat, twisting during entry/exit of the vehicle, moving around a bit when the officer is walking/running, and so on. All of this motion can break down the individual fibers over time. The garment's expiration date is an approximation of how long it will take for the fibers to break down to the point of doubt when the garment is exposed to the typical amount of daily use.

If your garment is hanging in the closet most of the time, the expiration date is meaningless. If you wear it once or twice a year, the expiration date is equally meaningless. If you wear it less than daily, it shouldn't be something to worry too much about.

that's the way I understand it, but as I mentioned, I have no idea if this is true.