I agree about letting the child choose what they play.
DS4 started at barely 3, they took him because he could read well, had amazing eye hand coordination and long fingers. He has done amazingly well and has a wonderful creative teacher who keeps him on his toes. Right now she makes a lot of the work about super heroes! Before that it was little Einsteins, dinosaurs etc.
DS can now totally sight read music and the themed practice is fun. I suppose
If a teacher was willing she could find something to motivate an older child.
A Julliard graduate has offered to teach DS when he turns five. He says he'll make it fun too!