Quote Originally Posted by Wardog View Post
He might even be able to grow up on his own, through his own emotional development.

It's already established that your age in the afterlife depends on how you see yourself (Eugene being a "grumpy old man" even when he was alive and physically young).

And (at least by implication) people can mentally change and develop in the afterlife, or else they would be effectively be in stasis and and unable to learn anything about anyone or thing there.

So a combination of "learning through play", being taught by or otherwise interacting with people, and natural curiosity could mean that Eric mentally matures, and "physically" matures with it.
We should also point out that Roy was explicitly told that he only looks like he did upon his death because that's where his mind was stuck, and his appearance would change eventually.