It was my 6 yo I was referring to, he will be 7 mid-August. I sent him to kindergarten two weeks after he turned 5, against some advice including his kindergarten teacher (whom we knew as she taught my daughter).
He is doing really well. He is at the top of his class academically, is a social leader and has never been reprimanded for his behavior. All issues that come up as reasons to hold a young child back, as I am sure you know.
Where I see a difference is when I compare him to his older sister. Her birthday is in November, so she is one of the older kids in her grade. She was early to read in kindergarten, was reading at a 3rd grade level by summer, and was tracked for gifted enrichment and testing. My son is reading at the same level by age, but while he is considered a high achiever and has had enrichment, he hasn't been tracked for gifted testing. So is my daughter gifted or normal for age? Is my son gifted? I think so, not wildly gifted but in need of more challenging work than they have been offered so far.
Honestly though, I am pretty frustrated with the public school system in general and we are working on a plan to homeschool in the fall.