For auto-inflate PFDs there are two components, the CO2 canister and the baking soda "trigger". It is usually the baking soda component that seems to go stale earlier. A fresh one will likely fizz when it gets wet (if you are old enough to remember baking soda submarines you will know what I mean) and release the CO2, while the older ones will just tend to get mushy and might not work properly. You can still use the manual inflate pull-cord, but the components still need to be up-to-date to be legal, especially if you have a "C" on your boat.