"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 as "gifted", and placed on a social/academic pedestal.
People have a wide variance of ability and timing during normal development. saying "it's much less stressful to learn to read early" is arrogant and naive. It's extraordinarily stressful to a child to be told they are stupid/slow when they are simply not ready, and stereotyped and classified as a result.
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.
I think John Taylor Gatto pointed out that some children learn to read when they're 2, some when they're 8, and by the time they're 12 you can't tell the difference.
I met a man who was traumatized when his first grade teacher tried to force him to read, before he was ready. The lesson was put in front of him, and 1st-grade-him thought he was stupid for not being able to perform.