The honest answer is, I don’t know.
Celiac Disease: A Hidden Epidemic says that on average in the US it takes 9 years from the onset of symptoms to a diagnosis. I’m pretty sure it’s taken at least that long for me. Probably longer.
I have what would be termed silent celiac. I didn’t have the traditional intestinal symptoms of diarrhea, rapid weight loss, obvious malnutrition. If I had, I expect I would have been diagnosed a lot earlier. I had hormonal irregularities. The summer before I left for college, my father started having me tested to see why I had such irregular periods. They’d started when I was around 14 or so, but never regularized. I once skipped an entire year.
I had elevated prolactin levels. But that was all anyone found. I suspect that was the first sign we missed.