If she were standing behind you, being shy and not really enjoying herself, then I would think that you should wait till next year.
If I thought you should wait until next year, it wouldn't be because *I* was upset or that I thought you were outside the bounds of etiquette.
It would be because I would feel a little sorry for your unhappy kid, and for you, that it wasn't as rewarding an experience.
HOWEVER, I know enough about kids now to know that she could act shy and SEEM to not enjoy herself, and then three days later to chatter about it so happily.
So I don't care how old they are. Part of trick-or-treating is for me. For ME to get to see the kids who live near me, no matter how shy they are. And if they hide behind Mom or Dad at my house, *I* still get to say hello to Mom & Dad, and I get to say hello to *them*, and admire their costume, and give them candy. And in 3 months, when I wave to them outside, they might suddenly remember that I was that nice lady from Halloween, and wave back!!
Or, even if I never see them again until next Halloween, they might be braver, or might remember me, or *I* might remember them. Or even if I never see them again, wherever they DO go for trick-or-treating next year, this year's experience will be part of the continuity the kid is building.
Trick-or-treating, to me, is about building community. And my "community" includes shy or nonverbal children.