Kegs are perfect vessels to age in. The only disadvantage is that you're taking a keg out of rotation for a long time. Then again, it's not as if they're much more expensive than glass fermenters (which I'd recommend for aging over a plastic one, though not strictly necessary). Honestly, knowing what I know now, I'd have purchased more kegs are fewer fermenters over the years.
If you're aging for a long time, maybe just check every couple months to make sure it's holding pressure (more to the point, that it's all CO2 in there).