Different types of people age at different rates, but how old they look is often effected by their will, much like everything else. Take this page:
Sadako starts out feeling very old and depressed, but she gets younger as she perks up at the end. As Queen of Nod, she especially is able to affect how she looks depending on her mood, as she has the power to affect everything.
Lucy looks old at the very end, but she can change her head, so that was a choice she made. I imagined that her kind of people, the headless ones, had a sort of culture where it got to be kind of gauche to continue wearing the same kind of young head forever, or they’ll have phases where they look old for a while before returning to a younger look later. Or maybe old face is sometimes fashionable.
Nod also has thousands of sentient humanoid species, and they all have various quirks about them. Don was the closest thing to a Nodian human that the story had, but he still wasn’t human. So some of these species will live a few days, some will live millennia.
Don pushed her through the portal because he was the one Fiona most trusted, but also, Nod itself has a bit of sentience, and it has ways of influencing events so that the one it wants to rule it ends up in that position. I’d actually love to explore more of that someday.