I don't think I've ever heard of a case when it just showed up. It's hereditary so I'm sure you had it since birth. I was 6 (50 yrs ago) when mine first came to light. I was in school and we were told to color the American Flag. I colored it wrong and was sent to the Principles office where I immediately received 10 smacks with the paddle for defacing the American Flag. My parents we're not happy.
Mine showed up in second grade. Half way through the year, I started coloring the trees green and the leaves brown. My parents were called in for a teacher/parent conference. It was at that time I was given the opportunity to view those little circles with the hidden numbers in them. 
We figured out that the crayons had worn down enough that the labels have been peeled off so I couldn't read which crayon to use when.

