You know your child best. I would say that generally, it wouldn't be a good idea, though. I don't know your friend who took her 21 month old, but are you sure that she and he weren't an annoyance to the other movie-goers? Did she go to a matinee on a Tuesday when they were the only 2 people in the theater?
4 years is what I would suggest for "most" kids, for a kid movie. Some can do it earlier and are well behaved, know how to behave and what to expect, and actually enjoy the entire movie without issue. Some, even at that age, cannot. Not without being a disruption to everyone around them. Trust me. I've sat next to those kids before. Constant whispering, passing around noisy snacks, kicking the seat of the patron in front of them, etc.
Your initial reaction was that your daughter is too young. So I would stick with that gut reaction and wait. Rent it/get it on Netflix. It'll be out soon, I'm sure.
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Exactly Sherry! That's what I wonder about the friend who took her 21 month old and said he was fine. I wonder how much of that she was doing and thought it was ok.