Vox has a good article on the effects of smartphones on the brains of teenagers:
Is there anything that tells us there’s a causal link? That our media use behavior is actually altering our cognition and underlying neurological function or neurobiological processes? The answer is we have no idea. There’s no data.
The article talks about what data we have, the limitations, why the limitations matter, and what would fix it. Science is hard. Medicine and parents are in a tricky place as they have to make recommendations with imperfect data. The news is sensationalist.
Galahad wants to discount all science on this, of course. He might be an addict. (Take his phone away from him and he goes through the typical behaviors of an addict.) The non-causal link does say there is something going on with smartphones and kids, so limiting usage probably leads to better outcomes. Enough so, that it is worth at least trying.