You definitely need to know your kids. I took my niece to see the Nutcracker last winter, when she was 3. We got aisle seats near an exit, because I wanted to be able to take her out if it didn't work out. I was thrilled when she sat mezmerized through the entire first act and the first 3/4 of the second act. When, about 10 minutes from the end, she wanted to ask questions, I told her that if she wanted to talk, we'd have to leave, and she wanted to talk, so we left, and didn't bother anyone for more than the 15 seconds it took us to have that exchange and then walk out. Now she asks me constantly when we can go see dancing again, and also why Clara's brother was mean and took her doll from her. And I've told her about curtain calls and how if you stay until the end, you get to thank the dancers by clapping, and she loves that idea. I'm getting tickets to see the Spongebob tour with her next year, when she'll be 5, and I'm going to make the same preparations, just in case, but based on that experience, I think she'll love it and behave beautifully.
My nephew is now the same age she was when we saw the Nutcracker with her, and he's a chatterbox and a wanderer, so we're going to go see a minor league baseball game instead, since I think that'll be a better environment for him. It's not the age, it's the child and their needs and their personality, that dictates. I'll take him to see the Lion King or something when he's a bit older and his attention span is longer.