I've seen a few characters who have lived out the 100 years Link has been asleep, and in cases like Impa, they have aged along with it. The director seems to have had an experimental accident happen to stop aging, most likely to do with her putting Link in the Resurrection Chamber. Point is, she is a special case and everyone else has aged.

Zelda is supposed to be in Hyrule Castle. So it makes sense that she is a 100 year old woman too, aged and withering. All the footage of a young Zelda are Link's memories, and in the twist you find an old Zelda in the castle, who ends up dying.

Or, at least, if I was writing up a long, suspenseful story inter spaced with flashback memories to create an emotional attachment to a person you never see until the very end, I would sweep the rug out from under everyone by making the person completely different in appearance due to time passed and also take them out after reuniting with the character trying to remember them.

But I've only gotten as far as Kakariko, so I know nothing.