That didn't change in 2010 though, did it? None of that seems particularly different from 25 years ago, and their figures show this was clearly a sudden change

That didn't change in 2010 though, did it? None of that seems particularly different from 25 years ago, and their figures show this was clearly a sudden change around 2010-2012. The smartphone does feel like the only convincing explanation to me, because it's the only thing that happened globally around then which was a persistent change in the lives of teens.