@jellofishxo Here is a study that shows increased performance at a 3rd grade level for kids that start kindergarten later:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2140009/. However, that's at 3rd grade.
I'm trying to find a link to a more comprehensive study which covered longer time spans, showing that the results evened out. They did mention an exception, that early learning programs that were truly top-ranked (like the kind with the $100,000 deposit and 6-year waiting list) did show an improvement.
The link was posted in the r/sciencebasedparenting sub. That sub used to be a great resource, but now overrun with anecdotal comments rather than actual studies.
Trying to do a Google search for this is like hashing thru 100s of Mommy blog results, which is never a good sign for getting to the science behind it.
I think qualitatively, you have to ask if you feel your child is ready. Mostly from an emotional standpoint. If you feel he needs more time before you set him into the wild, then by all means. If you feel that they are good to go and can handle the environment, then they should be given that chance.
There were some comments about older kids being bored. I was ahead a year (skipped kindergarten), and was still bored out of my mind. I wouldn't let that be a deciding factor.
Just keep getting feedback from their daycare, and just see how you feel. Stay away from the Mommy blogs on this one. Find some good peer-reviewed data and please update with anything that you find.