I took DD to see Monsters University yesterday. It was her first movie and she will be 3 in 2 weeks. She only made it about halfway through before she started getting antsy, going to bathroom, talking loudly, complaining about being uncomfortable and tired and wanting to go home. But...she also doesn't sit through a whole movie at home either so I wasn't really surprised.
We just took DS to that movie. He is 3 and a half. He did great and sat through the whole thing. We did get him some snacks and a pink lemonade, and he munched on them throughout the movie.
My ILs started taking their daughter around 3 to the movies - but they go to a theater that has tables/booths an serves deli sandwiches and such so there was more room for her to mull around a bit
We took DS to see Monsters Inc in 3D over Christmas break...he was 3 months shy of 3 years old. He did fine, just curled up in H's lap and watched quietly.
We took our oldest to see his first movie at 3, but he could definitely sit through a full-length Disney at home at that age. Our second? No way. I'll be surprised if we take him before he's 3.5 or 4, just because he doesn't have the attention span to sit in a theater for that long. Monsters U is also on the longer side--it was over two hours including previews when DS 1 and I went last week.