What age can you expect a child to happily sit through a movie at the theatre and do you have tips to make this a good experience for both children and parents (e.g. which movie to pick, what to do if they are bored after 5 minutes, etc.)?
My kids all did fine from about the age of 3. The only real way to know which movie to pick is by seeing what they like at home. My kids all have wildly different taste in movies, so usually I take them one at a time. Sometimes they get bored during exposition, but just require a quiet reminder from me.
You can't really judge it by how much they sit still to watch a movie at home. At home, my six year old son only lasts around 10 minutes before he wants to get up and act out the parts or run around or something, but in a theater he does pretty well because I'm there to refocus him.
This varies so much from child to child I'm not sure their is a useful answer other than just to try them. In a worst case you will have wasted a few pounds/dollars/euro etc and have to leave early.
My eldest enjoyed films from about 4, but my middle daughter was not a fan until she was about 6. I remember taking her about her fifth birthday and she wandered off under the seats in front for a while. We weren't worried as it was an empty cinema and we could hear her, but when she returned chewing on a sweet she had found we decided not to take her back until she had a better attention span. My youngest has watched films since before she was 3 and could always concentrate for an entire film. I think she picked this up through exposure via her siblings. . .
What age can you expect a child to happily sit through a movie
It depends on how much media consumption they've been trained to handle. My 3½ year old watches about five minutes of TV per day, just one episode of Barbapapa before bed. He wouldn't last long in a cinema. On the other hand, some pre-schoolers spend between one and three hours per day in front of the TV so for them a cinema trip would be nothing out of the ordinary. (Effects of that is discussed here and here.)
If you think there's a chance your kids will be bored (or mentally overloaded) after five minutes, then they're not ready for the cinema. They need to be able to concentrate (not start fussing) for at least an hour. As DA01 said, be ready, prepared and willing to leave the cinema at a moment's notice.
which movie to pick
Adhere to the advisory ratings; they do serve a meaningful purpose. Pick a movie that is truly aimed at the age of your kids. Too often I see parents with very young kids going into Disney-category movies that are way beyond them. You're not doing them any favors when choosing titles aimed above their age.
Pre-screen the movie yourself. Go and see it yourself, all of it, to make sure there aren't any unwanted surprises that you'll have to sort out later. (Try this one on for size: Did you know that Disney's Bambi essentially begins with the death of Bambi's mother?)
It depends on the kid. My 2.5 year old could sit through a movie with no problem. My 5 yr old neice couldn't.
Something to try would be a movie night at at, set up ad much like a theater as possible (lights low/off, popcorn, drinks, etc.) And see how they do. That's what I did with mine.
It's not a perfect method, but could give a base idea if what they would be like.
It depends if the child watches or is interested in TV first of all . And if the child will stay still and quite and not get bored after five mins. Try short movies at home first and see if your child is interested ,try also to see what kind of movie your child likes and go for similar ones at the cinema . Every child is different so observe your child and you will know when he or she is ready !