as early as they're willing.
I started when I was 5.
Just don't make the mistake my folks did and turn practice time into work. They'd set a timer and make me sit there sawing away for an hour a day - got to where I no longer enjoyed music and instead dreaded having to take an hour off of other stuff to play. And before they started that policy, I'd play the thing at least 3 hours a day, just not all at the same time. Ruined the violin for me.
Fortunately, I have lots of other instruments I still play
