I heard there's a policy of kids under 17 past 8pm had to be accompanied by a adult over 21? Is this true and is it in all amc theaters or just this one?

Yes, this is true. I don't know if this is true for all locations but, it is for this one. It says it at AMC's official website for this theater/location. https://www.amctheatres.com/movie-theatres/los-angeles/amc-rolling-hills-20 Using that link, scroll down the page to "Other Policies" and tap the question mark for more info.

I think it's after 10

Not sure. Id definitely call them directly to find out any policy related questions like that