This question asks "What age can you expect a child to happily sit through a movie at the theatre"? The answers may not necessarily indicate at what age all the overwhelming sound and light stop being harmful for the kids.
AAP recommends For children ages 2 to 5 years, limit screen use to 1 hour per day of high-quality programs. I'd think this applies just as much (if not more) to theater screens and surround sound. But I haven't been able to find reliable source indicating this. I have found a lot of anecdotal evidence indicating for what kind of movies, and at what age kids can sit through the movie or enjoy it. But that is no indication of whether or not it is harmful to them.
So, When is it harmless to start taking a child to the cinemas? Both to their health and behaviour. When can we expose them to this level of sensory overloading?