Genetics is a big factor
but stress can induce grey hair pretty quickly. For evidence I point to the various presidents of the US. In four years they typically age quickly- Obama, for instance, has far more grey now than he did on taking office, and Bill Clinton (and Hillary for that matter) were both looking very old compared to 1992 when they left the White House. Jimmy Carter aged dramatically in just four years.

(Then again, I have to admit, my only grey is in my beard and there's not that much of that.)
( The Resident Loon Not a demographic. Do not measure., Wed 19 Sep 2012, 6:25, 1 reply)

My once bushy black beard is now a white - I'll be gandalf soon.
Those who don't know me very well have said it looks distinguished.
(Misery McUglywife an attention seeking sociopathic fuckstain., Wed 19 Sep 2012, 7:00, closed)