Is the appearance of gray hair due to stress considered a reward, and is the reward for gray hair in Islam exclusive to Muslims?

In Islam, gray hair is a light for the Muslim on the Day of Resurrection. It expiates sins and elevates ranks, whether it results from the hardship of striving in acts of obedience or is common among Muslims. A Muslim should be patient with worries and the graying that follows them, for in this there is expiation for sins and an immense reward beyond reckoning.