I don't know, the voice he used in the Robot Chicken clips I saw doesn't really sound like a twenty-year-old. It sounds like Luke Skywalker, but that's more because it has distinctive Luke Skywalker traits than because it sounds like someone that age.
Plus, it sounds too artificial. I kinda think it's impossible, no matter how good of an actor you are, to pull off genuine-sounding emotion while making such heavy modifications to your own voice. Which is probably why most of the KH characters don't sound all too different from their VAs - and any degree of artificiality they do have drops out as soon as it's necessary for them to emote realistically (see: Axel in 358/2 Days).
...come to think of it, it also might have something to do with the tradition of choosing actors with live-action experience for the leads rather than VAs with more animation experience. =P