There is an older study that showed the correlation is to the average age of the household you grow up in. So the first born starts life with two adults, the next with two adults and a child, etc. How that works in the last couple of decades with kids farmed off to daycare centers full of kids, I don’t know.
That explains a lot. I grew up in a house with my mother and her parents, and she was their youngest, so when I was, say, 2 years old, the average age in our house was about 38, if you figure me into it. And 51 if you don't.