We started at 3y/o. With that said we did so in a controlled way. So the original plan was a tablet but for a fraction of the price we got a very basic smartphone with no service meaning just at home with internet. We started with a max of 20m once a week to make it kind of special but nothing something to become crazy about. Over the years we have given more time for things like chores with a max of 2h a week typically stretched out over the week and some times we would just have a lazy day with it so all 2h in one go. We knew it was tech that she was going to need skills with so we don't count things like learning math or enligh against the hours allowed. We began practicing how to text and make calls with our phones and routinely make blocked calls when she's using one of our phones to see what she does and to encourage open communication with us. For us this method has worked amazingly but I know this is not the norm for most.