What we've all already known - but now it shows that the earlier you start, the better. Before age 7 even! "The study provides strong evidence that the years between ages six and eight are a "sensitive period" when musical training interacts with normal brain development to produce long-lasting changes in motor abilities and brain structure."
From Science Daily:
If you started piano lessons in grade one, or played the recorder in kindergarten, thank your parents and teachers. Those lessons you dreaded -- or loved -- helped develop your brain. The younger you started music lessons, the stronger the connections in your brain.
A study published last month in the Journal of Neuroscience suggests that musical training before the age of seven has a significant effect on the development of the brain, showing that those who began early had stronger connections between motor regions -- the parts of the brain that help you plan and carry out movements.