Eh, they do age but a spare capacitor sitting in a likely climate controlled building and not being used isn't going to age nearly as quickly as the one in use likely in the outdoors. Will it be as good as a brand new one? No. But it will be damn near as good and it will be on hand when you need it.

At the same time though, if a motor kills start components often enough that you need to keep a spare on hand then there is something wrong with that motor or your power source.