There's so much stuff online that's so obviously going to be harmful if kids/teens are constantly exposed to it - porn, violence, fake/catastrophised news, angry/hateful tribalism, bullying/harassment, dangerous stunts performed for likes, etc. And social media algorithms can bring a lot of more outrageous content together into one endless feed.
It shouldn't really require a scientific study to know that the longer you can keep kids away from that stuff, the better. Yes, there's also amazing educational resources online, but they can be accessed via a filtered internet connection on a home PC, where parents can have some idea of what their kids are doing online.
If kids need phones for communication, maybe an old dumb Nokia that can make calls+texts, has no internet access, is practically indestructible, and isn't a theft magnet might be a better option than a smartphone?