Hair goes gray because pigment cells in the hair base at the roots of the hair stop producing melanin.
It doesn't matter if you are fair-haired or dark-haired you have the same chances of getting gray hair. However, it is more noticeable in darker haired people.
Graying of the hair generally starts at the age of around 30 for males and 35 for females however since the onset of this phenomena varies greatly from person to person, age is not the most accurate indicator.
The graying of hair appears to be genetically determined but the connection isn't at all clear.
So for now, your hair turning gray is just one of those mysteries of science that we have yet to solve. But salt and pepper hair always did look distinguished.
salt and pepper: 黑白相间而呈灰色的