About 17 (22 now). I’ve always got a few, but you wouldn’t notice unless you’re really examining closely.
In my early 30’s. About the same time I discovered the receding hairline and unseen (by me) thinning on the top of my head. Not many, just a few that I pulled out, but more appeared.
Late 30s after a lengthy and tumultuous love interest with a little lady I found I had major amounts of gray hair.
I blame her.
Late 20’s. My beard went salt and pepper, closely followed by the rest of it.
When I was 17–after baking a batch of cookies and washing my hands up–I noticed in the mirror that my hair had an unexpected–silvery ‘shock’ to it. I was quite taken aback as you can imagine–since I was fairly young to be getting grey hair.
Then I realized it was -flour-!
16 or 17.
I got a few when I was pregnant, but they fell out afterwards and never came back.
That’s my story and I’m sticking to it.
- I pulled it out, it never came back.
I was born with a white/silver streak in my hair and I’ve had it ever since. It’s not that much – maybe about 20-30 strands total – but if I part my hair just right it’s visible. I kinda like it. 
My brother and sister also have small silver streaks in their hair. It must be something in our family.