It's more than just achy joints and arthritis, researchers sayDuring this year's baseball playoffs, Chicago White Sox outfielder Ken Griffey Jr., 38, threw a picture-perfect strike from center field to home plate to stop an opposing player from scoring. The White Sox ultimately won the game by a single run and clinched the division title. Had Griffey been 40, it could be argued, he might not have made the throw in time. That's because in middle age, we begin to lose myelin — the fatty sheath of "insulation" that coats our nerve axons and allows for fast signaling bursts in...