Time passes differently in the different layers.

Here's an excerpt from a interview with Kevin Feige.

Peter: So there are layers that time does not pass by?
Kevin: That’s what they’ve told us. They’ve told us that it, time and physics and space work very differently down there. That was a big question during the development process. And as usual, you’re the first one to pick on it a little bit, which is… should she have aged or not? And we felt that new Quantum Realm, we could justify either one. But that ultimately you wanna have an emotional reunion with Michael Douglas, an emotional reunion with Evangeline Lily and our first instinct had always been specifically now Michelle Pfeiffer from that first movie. It felt like it should be somebody who’s the right age. As opposed to Michael Douglas with somebody who has not aged. Or Evangeline Lily connecting with somebody who’s not aged. That just adds another layer of sort of sci-fi weirdness. Yeah. And we’re not afraid of sci-fi weirdness. I love it. But not in that case.
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