When I started school in PA, the cut off date was the end of February of the following year, and my school district didn’t offer kindergarten. So, a child could start first grade at 5. I did.
People weren’t yet holding their kids back for added advantage, but they should have.
My oldest daughter’s birthday is at the end of September, just making the cut off date for NJ, where we lived when she started school. We sent her because she could already read, but when we moved to California, she was the youngest one in every class. In California, some of the kids who had been held back by their parents, were two years older than she. It became a real problem in sixth grade when some of the kids started to mature and get involved in more teenage problems like smoking marijuana or drinking alcohol in the girls’ room.
As a September baby, nowadays I would be held back a full year because of the Sept 1st cutoff date in my state of PA. That would mean not graduating HS till almost age nineteen, which is too late IMO.