Plastic ponchos, and rain suits for that matter, are all iffy after aging a few years. Kept cool, away from chemicals, not compressed too much, and with a bit of luck they can stil be good to go after years of storage. On the other hand If seen them weld themselves into a useless wad of plastic, partly dissolve when exposed to some solvents, and turn brittle and useless.

USGI versions, and better quality coated nylon versions, are much more reliable in the long term.