The acceleration point for both age groups studied is 2012. What happened that year? The article doesn't try to answer this. Might be mentioned in the study I suppose.
Instagram/Android was 2012. (Instagram iOS was late 2010). But not just Instagram; 2012 was about the time that social media really started adopting the dark patterns.
Possibly (probably?) a coincidence, but it did look like broad changes to how reading was taught started to land in 2010-2012: https://www.edweek.org/teaching-learning/common-standards-dr...
The real culprit is probably more in line with far more alternatives to reading for entertainment.
My guess is smartphones hitting a point of increased adoption. In the "good old days", phone games were honest and not addiction-inducing adware..
Anecdotally, 2012 is when I got back in to reading for pleasure, as a 16-year-old. I had no friends though, and thought someone cute might see me reading and become interested in me.
Prior to that, I stopped reading because video games were easy to get lost in endlessly. At the time, I recall I was probably playing a lot of League of Legends, TF2, Minecraft, and probably some others -- all of which I felt I could pretty much sink an infinite amount of time into, at the time.
Kinda, the measurement points are 2012 and 2020, so the decline is somewhere in that eight year period (birth years 1999-2007 and 2003-2011). My guess is phones/tablets strike again.