There is always a genetic aspect (not hereditary) that can cause premature graying. It is unfortunate that your beard is graying despite a healthy lifestyle. And I am assuming that you still 'continue' to live a healthy lifestyle...a healthy lifestyle needs to be 'followed routinely' to keep diseases at bay. If you were following and you stopped then that again could trigger the premature graying gene. And above all this I would suggest you visit a dermatologist to know what you can do to prevent further graying. Unfortunately you might not be able to reverse the damage but you could prevent future damage but only after being thoroughly examined.