There was research where some kids had their phones on their desk TURNED OFF (the ultimate in "restricting access") and they still did significantly worse on the test. Why?

Even when phones are off, kids are thinking about their phones. About the texts they are missing. About what's happening on social media. About who they should be texting. It's a major attention destroyer. Consequently they never learn how to "be present". Realize their brains are growing the fastest right now, setting them up for life.

Want to combat the cultural ADD epidemic? Don't give kids phones. Teach them how to be present in the real world. That will set them up for success in life. When they do get phones later on they will easily learn how to use them and can with on those phone skills then. When younger have them work on:
1. How to be social, particularly if you don't knew everyone around you
2. How to be engaged, even when everything isn't as engaging as a screen.

Those 2 skills will set them up for success