My ds 11 has been watching some 12/12a movies since he was small, Indiana Jones, the hunger games etc, more recently the marvel movies (all 5 million of them), and Cruella.
We watched shooter (Mark Wahlberg) last week, which is a 15, which he enjoyed, it's moderately violent, but relevant to the plot (which makes some good points about good vs evil and political corruption), and we've been watching Gremlins every christmas for years. He watched poltergeist last halloween, which is the only horror movie I've let him watch. He's asked to watch game of thrones recently, but that's a definite no from me (I'm a big fan of the show, so know just how bad it is, sex scenes in particular).
I tend to watch something first, alone, then use my own judgement (if I haven't already seen it). I did this with stranger things (TV 14), and ended up getting hooked myself. It all depends on the individual movie/show.
My other rule, to encourage reading, is he can read the book version, of anything he wants, whenever he wants/is capable of understanding.