I've never really given any consideration to the first possibility, but you're right that there's no definitive canon evidence one way or the other. Now that you've raised the question, I can think of a variety of circumstantial elements supporting each position.

Mostly it just doesn't make any sense to me that it would make them younger, unless there was something more complicated involved. That's simply because being brought across arrests aging but doesn't reverse it, so it's a big leap for me to think that whatever causes that would go further in a living mortal who was otherwise not being changed as much. But now that you've got me thinking about it, not entirely out of the realm of plausibility.