Really depends on how old. If it got dirt and such embedded in it, it has not be stored in climate controlled area most likely.^^^^^^^ I agree. Climate controlled is vastly different than in use. However, everything ages and breakes down, dirt gets imbedded in fibers and accelerates breakdown as do common chemicals and UV Rays. Mfrs can't control the vest/plates exposure wear after its sold. Expiration dates are Hard regardless of use/storage. There's vids with expired vest taking hits within Level rating and still working, most to a standard slightly above Level but I wouldn't volunteer to be the ballistics test dummy. Your call and liability to continue in service.
I'd wear a 15 year old vest the was climate controlled if I had no other option. I believe there have been out of date vest tested on youtube and pass but I'm not going to post any links as I dont want to sway your thoughts.....
Off topic but and different material but this also makes me wonder how good the airbags are in 20 year old cars are with all the heat waves we have had here in Florida.