Regarding the term of "death" in the KH-Universe, I'm quite confident to say that it exists. If not, why did Master Xehanort go through all the trouble of finding a "new vessel" to escape his aging/dying body?
The whole Heartless/Nobody situation seems to be a special case of "existence" which was caused by Ansem's/Xehanort's research in the first place. The body splits up with the emotions/humanity of a person. Since we didn't actually see a person which became whole again yet, we can't actually say if the person also stops aging like in the Realm of Darkness. My guess is that they don't, but that's yet to see.
So I guess by making Xemnas desintegrate and joining with his Heartless, the chance of changing Xehanort/Terranort back to Terra seems quite good. What interests me is the fact if they let Terra age the 11 or 12 years he spent as Terranort when he's transformed back, and if Ventus stopped aging too, like Aqua.
Since it's the former Land of Departure and sleeping in a delirious condition seems not to be an issue (look at Sora) I'd say he should've aged, making Terra around 30, Ventus around 25 and Aqua staying around 20. Though I highly doubt Square would go down the logic road, forced to spend money and make at least one new character model. ^^