The source article provides a lot more info
https://ed.stanford.edu/news/stanford-g ... ndergartenThis article says that there is NO demonstrable difference in financial or other success measures for adults, and the test in question only measured up to age 11 - and only looked psych measures (not intelligence, performance, grades etc)
Then it goes on to say that what they *really* looked at was month of birth - because it was done in Denmark, and there the cutoff date is very strict.
And, in Denmark, the children do have "access to" preschool (okay - they're FORCED to go)
There have been many studies over the years looking at month of birth and various measures of grade school success.
What they're really looking at is not delayed entry for everyone, but whether there is an advantage to being a year older than one's classmates.
Since DD has no classmates, I'm not worried ; )