Even at my suburban public school, there are many kids who are held back. We have a December 1 cut off, but there is a kid in my son’s grade this year (seniors) who moved here after K, repeated K and turned 18 in June before senior year.
One of my sons has a June birthday. He graduated with his class. A friend who is a month younger graduated a year behind, as did 2 other friends with October and November birthdays. Another friend with an October birthday was in the correct grade but more than a year younger than some of his classmates.
My oldest has a July birthday. When he was in K, everyone tried to convince me to hold him back. I refused, in large part because my D is 14 months younger and I felt that having them in the same grade would be psychologically damaging to him.
As an interesting corollary to the holding back athletes story - two of the boys I mentioned above are musicians. The mom of one, who is an only child, said she and her H decided to hold him back when they realized he had musical ability on the theory that being a year ahead would give him a heads (or a hands) up in terms of motor skills and coordination as well as ability to concentrate. It must have worked, he became all state on his instrument and is in college studying performance. His maturity relative to his classmates obviously helped.