All of this is not to say that you shouldn't begin your child younger. I plead guilty for beginning my two youngest sisters at ages four and three. :) Hearing them pick out melodies was fun, but when they began to put chords to those melodies (and the younger one SUBSTITUTION chords! [yes, she was three]), I couldn't contain my excitement, and lessons began!
The bottom line is to pray and seek the Lord as to when He would have your child begin. There is no "right answer" for your child but the one God gives you the peace about.