I don't think you can start too early BUT the child has to want to do it themselves and have a desire to learn. I started learning the violin at the age of 4 and was never pushed by my parents into practicing and learning the instruments. I agree that the best thing is to sing with your chid, introduce all various different types of the music. The child has to enjoy what they are doing. You may find that if you try to push the child in a particular direction they will rebel against it and not want to do it you may find that as they get older they may want to learn an instrument that is different from their parents.