I think our son was around 3. I can't quite remember EXACTLY. If it had not gone well, we would have taken him out of the theater, but it wasn't an issue.
It is important to explain to them in advance what is expected of them at the movies, though. Consideration for others especially (not talking/whispering during the movie, crinkling snacks throughout the movie, etc). And also what to expect for them to experience (darkness, dimming lights when the previews begin, a little bit of noise from opening snacks during the opening moments, LOUD stuff when the movie gets loud, possibly hearing rumbling from the show going on next door, etc).
Our kids (both of them) always were great! They get annoyed by boorish behavior by adults in the theater, but are usually well behaved enough to keep it to themselves until we leave afterwards.
I remember a few years ago taking my 2 kids to see Where The Wild Things Are. My daughter was 8 at the time. She was miserable throughout the entire movie, because it was a packed theater, so nowhere to switch seats, and she was seated by a mom with 4 kids---NONE of whom apparently had been instructed how to behave in a theater. Several of the kids were talking, asking questions of their mom during the movie. Asking for snacks. Mom was OPENING crinkly packages of stuff every 10 minutes.
It seriously was the kind of scenario, where you wonder why the heck she brought the kids, because they couldn't have cared less about the movie. They were bored, and it was evident. She was opening snacks non-stop to keep the occupied. The kids were probably in the 4-6 year old age range, and were not behaved appropriately for a movie situation.
Do I sound snarky about it? Too bad. I spent $50 for me and my 2 kids to enjoy the movie, not listen to her kids make noise for 90 minutes and ruin the movie for my daughter. :( Yes, it was supposed to be a "children's movie"... but that doesn't mean bring any and all kids, even if they ruin other patrons' experiences.
Teach your child early what is appropriate behavior in a theater, and what is not, including the difference between the advertisements at the beginning and when the actual movie starts. Talking, asking questions, rattling stuff, standing up and down, kicking the seat backs, etc... is rude to other patrons who have also paid good money to enjoy the movie. And I really don't care if it IS a "children's movie". My kids are annoyed by other misbehaved kids (and adults) whether it is an animated movie like Toy Story, or a more mature movie like War Horse. They are there to see the movie. If your children aren't, don't take them.
Wow, didn't mean to go on such a rant! My husband and I love movies and so do our kids. If my kids couldn't behave, then I would still go to the movies, but I would leave the kids at home with a sitter. I just wish more parents did the same. If your kids CAN behave... then welcome aboard!!