Do bullet proof vests expire

Yes.

If the vest has not been shot or stabbed or otherwise forceably attacked, the synthetic fibres in the vest will slowly degrade.

Most manufacturers suggest at maximum lifespan of 5 years in normal use. Storing the vest in a place with high humidity, constant exposure to direct sunlight and moderate to high heat will also shorten the lifespan of the vest considerably.

The above applies to most vests sold for civilian and police work in the USA. I do not know the standards applied to US Military equipment, but it would unlikely be any longer, and likely much shorter due to the considerably tougher environment most military vests find themselves used in.

After ANY hit or stab, the vest should be replaced.