As someone who has grown up and experienced being chastised for doing something, "childlike" such as watching cartoons, playing video games, etc., I can give a first-person account of the relative difficulties of self-expression through a medium.
Not to mention how a lot of my friends around my age and younger still have to put up with that nonsense constantly, which is anecdotal, but I think that's kind of the point.
So you're right, millennials didn't invent "having fun," but it sure is annoying having to justify it to folks on their way out of this mortal coil on a regular basis. Which, to me, is the entire basis of this thread and discussion in the first place, lol