yeah, we actually started when she was three.
She got actually got hooked when we caved and gave her a phone on a flight from Europe to Costa Rica.
Right after that it was all but impossible to take the phone away and we struggled for a long time (~1 year) to contain screentime. I think she was around 3.5 y when we decided we were going to go from 15 minutes a day to complete zero.
Like @vrod said, it's rough at the beginning and you have to be super present to be with them, but also allow for boredom to come in and not fill the boredom with some exogenous stimuli.
aka have a spine when parenting.