I really want him or her to be a great pianist, so what's the best earliest age to take lessons.
Is 4 good?
And yes, I know, you're going to say "Don't force it into him if he doesn't like it."
Of course I won't, but if he does like the piano, I want him to take as many lessons as possible at the earliest possible age.
I was told that you can start them when they can reach, but it really won't put them ahead of a child who didn't start until around 8 or 9 because they way kids learn. If your child is exceptional, then he'll pick it up sooner, but most likely, he'll be playing the same stuff until he's 8 or 9.
Call a teacher or two and talk with them. Find one who is a certified music teacher, not just some neighbor or friend. Make sure they're excellent with children and have a background check done. If it's fun, your son will enjoy it, but if it's a chore, he'll fight it.
Another thing to consider, is both of you learn together. He could start younger because he'd be with Dad and you'd learn together.