User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-3146930-20130105054140/@comment-5774152-20130105180315

^I guess I never thought about that before. I would think that Hazel would age seeing that she only came back from the dead and would not have aged in the Underworld. Since she is no longer there I would think she would age now.

Think of the Underworld like the Lotus Hotel and Casino. You go in there and you don't age but time still goes by and you just don't realize it. Then when (and if) you come out it's way more into the future then you expected. But would continue aging like Percy, Annabeth, Grover, Nico and Bianca did. But who knows, I might be wrong. Just would make sense to me that way.