I have 2 daughters that were born in August. The first one, I sent to K when she was 5.... so she was essentially the youngest one there.
She did ok the first few years, but it did reach a point where she was struggling.... when we moved at the end of her 4th grade year, we had her repeat 4th grade (with her full knowledge and consent) at her new school, which was also 1/2 the country away.
For her, that was the perfect time to do it. It gave her a chance to catch up. She just finished her doctorate in Microbiology at Johns Hopkins School of Public Health, and just started a job as a senior contamination engineer at NASA.
The second daughter, we delayed her entry for a year... she was just a LOT different than her big sister. Unfortunately, she still struggled... she had a lot of problems with ADHD, including the social aspects. We never put her on medication.
My point is, you never really know how it will turn out....... redshirting the second daughter didn't really help that much..... but that was because her ADHD is what she really struggles with.