I think the keys to long-living IPAs are to have them well-attenuated from the start, to keep oxygen out of the works, and to keep them stored cool.
American barleywines I tend to overhop, knowing that they'll fade back into balance over the course of several years, but most modern IPAs are essentially running beers made to be consumed quickly. Historical recipes can be different, but most people today want to enjoy the freshest hop character possible.
If you're getting additional sweetness, I'd probably suspect oxidation first. Or just too much crystal malt. Unless you're going for the baby barleywine kind of IPA. Nothing wrong with that, but it's more of a keeper/sipper than other types.