It's a hard thing to say "when it began."

I had always noted mine to my early to mid thirties when my oldest said, "Daddy's got a hole in his head!"

--But I can tell you when she said it I didn't find it amusing at all, not at all. It just kept creeping through my 30s, then the 40's, and 50's, and now I can say, it should have been shaved either in the late 40's or 50's. But I hung on, until it was just too obvious to ignore any more.
My come uppance and the point of this post was brought to the forefront again this past Easter when one of my oldest friends, we literally met our first day of college. We're driving around, and he asked if I was going to grow it out again. Obvious answer "No!", duh. Then the stomach punch, he said, "Well you did start losing it in your twenties, right after we got out of school!" I just had ignored it, but you know, that was a shock, not quite painful, but a shock anyway. Damn, I really was 10 to 15 years denying the fact I should have shaved it.

There's a happy ending though, now I'm sly--

The "take away" we probably don't acknowledge we're losing it until it's been going on for a LONG time. Sad, but true.