Some caveats to my claim. The thing (book? podcast? can't remember, so take with a grain of salt) was saying kids that read early (ages 3-4) aren't more likely to have a degree at age 35 when controlling for other factors.

The study you've linked is looking at reading ability at age 6, which may change things. I also couldn't find anything in the paper stating they had controlled for other factors. If a child with a parent at home reads earlier, and one that doesn't reads later, the difference in life success is unlikely to be related to the reading, but more likely to be related to them growing up in a household wealthy enough to have a stay at home parent. This wouldn't be effective data for changing a school system to focus on reading earlier.

To put this in context with what I was saying, if reading is pushed hard from age 4 and some kids do well and for some who aren't quite ready this causes a detrimental impact that causes them to be poor learners for life - well this would show in your study as reading ability at 6 years old being strongly predictive of reading ability at 42. Basically, I don't believe the study you've linked helps us narrow down anything because they don't seem to have controlled for anything.

I did try to find a study to back up my claims, or even disprove my claims, but I couldn't find a study that looked further than 10 years or so, and even then they weren't controlling for other factors.

Maybe our school system is failing because of a lack of data on how to do things better?