On Kids and Smartphones - Cal Newport

Newport digs through the data on this question and provides some context on the conclusions.

Over the last few years, a rough consensus has emerged that there really are significant harms in giving young people unrestricted access to the internet through smartphones. This is particularly true for pre-pubescent girls. This consensus arose in part because the main critiques raised during The Data Wars were resoundingly answered, and because, more recently, multiple independent threads of inquiry (including natural experiments, randomized controlled trials, and self-report data) all pointed toward the same indications of harm.

There is a full podcast on it too in Deep Life #246.