A very UN-scientific survey...
Age... well... I'm 55.
My hair started turning noticeably grey about 10 years ago (checking in some photos right in front of me). :cry My mustache is mostly grey now, but still has brown strands in it. My eyebrows are still definitively BROWN, with a few hairs turning grey.
I use no hair products AT ALL, not even styling gel... Stopped using Brylcream :rolleyes when I was about 15!!! :grinroll
If that helps.... :nerd