You start teaching them at an early age how society may see it as no big deal, but society is wrong and it's a very big deal. Show them through your actions that life is better lived without such distractions.
And be prepared for everyone to call you a conspiracy nut. But you eventually let that roll off your back when you see how happy your kids are just being kids.
What brutal modern life problems require smart phone with unlimited access?
Most problems we face in our lives haven't changed a zilch - its simply interacting with people for whatever reasons. You don't need phone for that, in fact you should first work on handling face-to-face interactions and only then move to remote ones.
Kids can grok how phone and app works in few weeks through and through and much faster than you or me currently, so that they are not missing anything if they start later. There is very little to 'learn' by further usage of it, just feeding addictions.
