There's a good article in the New York Times this morning about recent discoveries on the causes of gray hair. Whether one gets gray hair prematurely or late in life doesn't seem to have anything to do with a stressful or stressless life but is mostly due to genetics.
It's also interesting that scientists have looked, but have not found a link between going gray and real aging in the body. People who get gray hair earlier than their peers do not die any sooner. Gray hair (and baldness) has very little to do with premature aging.