at 40 or 41 - it's been very gradual but i'm closing in on 48 next month and that gradual increase every year for the past 7 years adds up!
not sure if i'll ever try coloring it when i'm more gray than dark - most guys i know who have dyed their hair usually stop, guess they weren't ready to accept aging and then later on come to accept it.
check your balls - it ain't just your beard and head that gets gray!
