In college but I also have black hair so they're pretty noticeable. My dad had a full head of grey hair at 45 and I'm on a similar trajectory. Thankfully I don't have bald spots or a receding hairline though.
For both it was two days after I financed my third luxury car. I scheduled it so that I make a bi-monthly car payment every 5 days. That way I never forget.
Got my first one about two months before I turned 30. Was going through a ton of stress and things that I didn't even realize which were laying the groundwork for marital problems years before anything came to the surface.
Fast forward six years, and I am divorced (almost) and have probably a handful of gray hairs but my hairline has been in full retreat mode ever since.
I've added to the collection of white hair at a slow and steady pace since then and now in my late 30s my hair is probably still 99% black but the white hair is very noticeable due to the color difference.
I'm 38 and basically platinum 'blond' at this point. Started with a clump in college and spread fairly quickly over the years. Only saving grace is that all the twenty somethings I meet are really jealous of my hair color.