You never know when set notation is going to pop up. Usually, you'll see it when you learn about solving inequalities, because for some reason saying x < 3 isn't good enough, so instead they'll want you to phrase the answer as the solution set is { x | x is a real number and x < 3 }.
If the skin really looks like the color of ash, a deeper more persistent gray or gray-blue color, this could potentially be due to a side effect of certain medications, with or without interaction from sunlight.
A small amount of black eumelanin in the absence of other pigments causes grey hair. A small amount of brown eumelanin in the absence of other pigments causes blond color hair. As the body ages, it continues to produce black melanin but stops producing the brown version, explaining the grey hair common in the elderly.