There is a "shelf life" on body armor but it is mostly a formality invented by lawyers wanting to repeat-sell on existing users. Body armor lasts a long time, especially vests using kevlar (yellow woven fabric). Spectra or Dyneema (white shiny fabric) also lasts long but I am not sure if it lasts longer than Kevlar. Zylon on the other hand is a no-go for me (good thing they are no longer being built).
Me and my friend got an old Second Chance Kevlar vest (PASGT vest-style with woodland camo and front overlap) made in 1987 (from really faded markings on the interior). We shot it with Magtech 45 ACP FMJ from a FNP-45. The rounds did not go though, not even 1 layer and it had 30 layers (we opened it up). So from my experience, ballistic vests are a good investment.