I think kids get busier as they get older. I fished way more as a grade schooler (6-12) than I did as a teenager (13-17). After elementary school, things get busier, more homework, more sports, dating, etc.
My heaviest fishing years were when I was single in my 20s, when I would get out on some kind of water a couple times a week. Now as a working stiff with a kid, I consider a couple outings a month as a success.
I think the point about changing disposable time/income across demographics is important. Younger generations have worse job prospects and higher student loan debt than previous generations, which forces young people to work longer hours to make ends meet. While fishing can be cheap or expensive as you want it to be, you can't fish if you have to work 70+ hours per week.