Legally, they have to be 13 to have a google account. So we get them a smart phone without mobile service at 13, usually a hand-me-down phone. Let them use it on wifi with Google Voice for texting. Lock it down, check it regularly, goes to bed with us at night. Then they can gradually earn social media accounts and mobile service over time. We have access to all their social media accounts. They need to have enough experience to manage it on their own by the time they leave home.
Remember, any security can be defeated, even the locks to your front door. So you can't just put monitoring software on the phone and forget it. We have found that we have to regularly check the phone and discuss it with our kids. It takes more work for you, once they get a phone.
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