My cousins earn their living as musicians (one plays keyboards in bands, another plays jazz piano, another is a session guitarist) and one of them started his son on drums at age 3. A year later, his son practices at least an hour a day without anyone asking him to -- it's the first thing he wants to do when he wakes up. I think he's inspired by seeing his dad onstage, practicing a lot, and giving lessons.
Well, I don't want to claim that every child who starts early will be ruined
I took a lesson a week and was expected to practice 30 minutes a day. I have a long attention span, I probably could have started earlier.
But as long you do something out of your own free choice, and not because Mom and Dad want it, or because Teacher wants it, or because you simply lack other choices, you will probably do well.
So you don't believe in making them go to school then?
... my first suggestion was that you should not give lessons to children until they are at least 6 years old. If there is any trained psychologist here, I think he or she could tell why this is not appropriate in general cases. After all, if you look around the world, most children in most cultures happen to start school when they are about 6. Not much earlier than that. There is a reason for that!
Actually I think school has damaged many children. I have a son with authism/ADD, and let's say that school from time to time has been a pure disaster for him. He will be 18 soon. He is intelligent. He has failed completely when it comes to grades.