One of the things my wife and I still argue about is smartphones. My view is: as much as possible, let's cut down on being on our phones in front of the kids. Turns out that's a lot harder than it sounds.
When she gets home from work, my wife lies down on the couch, takes her socks off, and looks at her phone, saying how tired she is. I get it, I really do. Driving over an hour to work in the morning and nearly an hour and a half back home in the evening — of course you're exhausted. But the problem is, she also tells our elementary-school daughter, "don't look at your phone late at night."
When your words and your actions don't line up, you lose your footing the moment your kid does the exact same thing. "You do it too, Mom." The second that line comes out, there's really no comeback. "It's fine because I'm an adult" just doesn't land.
If changing everything all at once feels too hard, try just one thing. Don't put the phone on the table at mealtimes. Just that one thing.
I'm not perfect either. But at some point I started flipping my phone face-down on the kitchen counter, at least during meals. My kid started talking a lot more while eating. Sounds small, but it changes more than you'd expect.
So when I say "get your kid a smartphone as early as you want," there's a condition attached. When you're with your kids, talk with your kids. At meals, at bedtime. When your kid talks to you, make eye contact and actually listen.
Parents have to be able to control themselves first, before a kid can learn to control themselves.