Earlier quoted context omitted.
The amount of text that a person has read improves their reading speed. So someone who has a big head start like that is going to have an advantage for more than a few years. It's true that eventually other factors might average them out, but why would you want to be illiterate for years and then make everything else you're trying to learn more difficult for years more? It's much less stressful to learn to read early…
You're missing the point. "illiterate for years" is of no consequence when we are talking about 5-8 year old children. The point was there was no discernible difference in children who learned to read a a variety of ages. In factory schools, if one is "delayed", that classification/label is attached to the child and becomes a part of their identity. Few escape it. Similarly, one who reads well at age three is held up…
Gatto's quip aside, there are actual studies that look at correlations. "Using third-grade national percentile rankings... into below (0-24th national percentile), at (25th-74th national percentile) and above grade level (75th-100th national percentile) groupings, we find correlational evidence that students who were at and above grade level in third grade graduate and attend college at higher rates than their peers who were below grade level in third grade." https://www.chapinhall.org/sites/default/files/Reading_on_Gr...
https://www.gov.uk/government/publications/reading-and-maths... found a 10% difference in earnings at age 30 for differences in reading skill at age 10.