They cite myopia as a specific clinical reason and while there is
some evidence correlating "near-work" activities (screen time, studying, etc) and nearsightedness, I personally think that's a weird way to slice a policy (especially considering the correlation isn't very strong). To be fair, it's not the only factor they cited, so I'm willing to concede it's an attempt at a "two birds with one stone" sort of policy.
The other major factor that was mentioned was addiction. I'm not sure there are studies on this, but at least anecdotally, a lot of people seem to think it is a problem.
Some other comments are also mentioning that this is more a restriction on companies providing services to underage audiences, and we do have a bunch of legal restrictions on how companies are allowed to interact with children; see COPPA.