The study:

https://journals.humankinetics.com/view/journals/pes/38/3/ar...

From an initial glance, it looks like the results are all based on self-reporting and that over an eight year period there were three reports: at the start, at the two year mark, and at the eight year mark.

I regretted buying my kids phones when they were 8 but they are 18 now and turned out just fine