Well, didn't you play with brain teasers and abstract shape puzzles and such as a child? Or edutainment software? Studying doesn't have to mean nose-to-the-grindstone.
I'm in my mid-30s. If there was anything we'd recognize as "edutainment" software in the 70s and early 80s, we couldn't afford it. Anyway, I have an average-software-engineer IQ, more or less, on tests in the 80s, and I can't remember ever doing much brain teaser or abstract shape puzzles -- those weren't interesting to me. I spent every waking hour I could reading past age seven or eight.
Anyway, do people study such puzzles? For such a consistent, widespread rise, you'd expect it to be pretty obvious that they do, but I'm not sure.