At 5, my oldest DD was on a princess hunt - had to meet them all. She liked them, but never was gaga over them. In fact, at BBB, she picked a Tink costume instead of a princess one. And she knew that Mushu was a person in a costume (but that's the real Mulan, right Mom? So cute.)
Anyway, at 6.5 she wasn't so into it, but her cousins were with us and it was their first trip and my younger DD was 3.5 and super into it. But without that, I don't think we would have done much that was specifically princess.
Now for our upcoming trip she will be turning 8 on the trip - we are huge character people, so we will be meeting Rapunzel and Tiana and any princes that we can find. But she wouldn't be caught dead in a princess costume.
I'd recommend going at 5. Even then, it may not be as magical.