Or for those who prefer grey hair 
I should say perhaps more accurately when did you NOTICE, you’re first gray hair.
I recall my first gray hair was on my birthday when I turned 27, I was looking in the rear view mirror of my car and saw it. AWWWW → PANIC → I was old. 
Of course it had to be there long before but I just noticed it.
So when did you first notice your hair was turning gray
I found one in the 4th grade. Just the one, and after it fell out I didn’t see any more for a few years, but I was finding them regularly by my sophomore year of college and started dyeing my hair to cover the gray by graduation.
I noticed mine just before my 30th birthday. I have super curly brown hair and found this long, straight, dead-looking hair just sitting in amongst the curls.
I made an appointment for my 30th birthday to have my hair highlighted. That way I could “cover up the gray” without having to “dye my hair”. It just mixes in with the blonde bits now.
I had three when I was seventeen. They stayed but no more came in until my thirties.
Now I have a bunch - my temples are the most noticeable.
Until I was about 20, my hair was blond. Then it started to darken a bit. Then when I was about 25 it seemed to be lightening again. I thought I was going to have blond hair again. Wrong. By the time I was 30, it looked pretty much like my profile pic.
Do gray/white eyelashes or eyebrows count? I have a couple of those, but I’ve had them for as long as I can remember and don’t yet have any gray hair on my head.
I had a few in my 20s but they exploded on the scene when, at 37, I got married. Pow!
My first one was at 14. By 17, I had maybe 10. At 31, I have two solid white patches at my temples and a fair amount of silver tinsel strands throughout the rest.
My mother used to find the occasional gray hair when she was brushing my (dark brown) hair when I was a child.
There were enough grays that I started coloring it to cover them by the time I turned 18. I imagine my hair is mostly gray under the dye now (in my 30’s). At least when I let the roots grow out too far it appears to be entirely silver/gray.
Hasn’t happened yet.
Of course, when I had my attack of brain fever at age 25, my hair turned white overnight.
First noticed one or two when I was seventeen.
The other day, I plucked one of the grays because it would not lie down reasonably with the rest. When I looked at it closely, I found that it was only about half gray. The end with the root was still brown. WTH?
Mid 40’s, no grey.
My kids pulled a gray hair out of my head on my 30th birthday and said “Mama, you’re OLD!” As if I weren’t traumatized enough.
I’m 26 and have been blonde my whole life, so one probably wouldn’t stand out even if I do have one.
Found a couple on the temple around age 30. Maybe a dozen on my hands & arms now (35). Also have a bunch of grey on my chest hair. All the ladies say it looks distinguished.
I started getting gray hairs about the time puberty set in, so around 10 or 11.
Oddly enough, my first gray and white hairs weren’t on my head. The showed up on my chest and arm pits first.
Only 28 now, and my temples are almost completely silver.
After starting a second career in my late 20s, I noticed some silver/white hairs at age 29.
It started when I was in high school. They are not noticeable at a distance, but you can clearly see quite a few of them when you’re up close.
I was born with a few white hairs, probably because my mom had a white streak in her hair. But considering that I’m almost 60, I’m not very gray.