In my late thirties I noticed flecks of grey in my hair when I was getting haircuts. It was like that for a few years then went away again and has yet to return

In my late thirties I noticed flecks of grey in my hair when I was getting haircuts. It was like that for a few years then went away again and has yet to return (clocked in at fifty last year). I don't recall life being especially stressful at that time. I went through an unbelievable amount of stress in the middle five years of my forties which didn't cause a recurrence. I do have these thick grey hairs in my beard though, that stand out among the thinner, wiry brown ones. Not sure what that is about.