Parenting

brains.

As with everything else, moderation is a good thing. If you spend all your time on a screen (or any other single activity), then you're not spending time doing other things, and thus depriving yourself of other important mental, physical and social activities. But I'm not at all convinced that this is any worse for screen time than TV (the bugbear in my own childhood), comics (a bugbear in the previous generation), popular music (always a bugbear in one form or another), etc.

I would guess that children and teenagers with attention deficits, mood problems, or reading delays are probably more likely than others to become addicted to screens in the first place. This may then be unhelpful in overcoming the problems; but I doubt that it's usually the cause of them.