My 11-year old (who was 9 when this all started) is probably better than anyone else in the family at wearing his mask. He even wears it for hockey practice. Granted we invested in some decent masks early on and they have been well worth it.
I am in my first week of teaching 7th graders, and I haven't had a single complaint about the masks from my 170 students. They wear them in class, and they wear them most of the time outside unless they are eating or drinking. Us teachers probably complain the most. I hate it, for sure. But I also try to remember that grocery store workers and many others have basically been living in these things for the last year and a half, so I can handle it.
My wife has more problems with students in High School. She has a whole group she calls the Trumpsters that driver her nuts with complaining about the masks. Keep in mind, my wife is a lifelong Republican, but she still has to do her job and some of these kids are a pain in the rear.
I'd imagine the younger the kids are, the more lenient the policies are going to be when it comes to masks. But, like wearing pants, using the bathroom, and washing your hands, you can work with little kids on the skills they need to function in public.