I agree with Hesperis about picking the right teacher but I feel that it's as, if not more,important to pick the right time. Many children are given musical instruction because it's what their parents want. I wonder how many of those kids grow up with a love and appreciation of music, borne out of their understanding of musical theory, and how many grow up feeling music was a chore. As an adult the choice to take up further education is usually driven by a desire to improve an existing talent and is therefore likely to bear fruit but to a child, making the same headway would depend entirely on the teachers ability to create a desire and show the fun side of making music before getting bogged down in scales etc.