After moving back to Finland from the US, I noticed that seemingly everybody gives their kid a SIM-equipped phone at age 7. That's the age when school starts (a year later than in most other countries), and a lot of children become fairly independent quickly, e.g. walking or biking to school on their own. It's just too convenient to be able to call and track the child by the phone.
In America, the smartphone seems to come several years later. The article here mentions that some parents want to hold it off until 8th grade!
The overall cultural difference is so great that this probably won't make for a valid experiment on mental health impact — but maybe there's something American parents will be able to learn from Finnish-style phone parenting practice eventually.
(Edit — a small observation from the Finnish kids. Their leading social media is WhatsApp Stories because everything else requires you to be 13+. A surprising data point in favor of Meta potentially capturing a chunk of the next generation.)