We have a Sept 1st cutoff date and our son has a late July birthday, and we chose not to send him.
When he was 3 he began preschool for 2 years. At first he was right in line with his peers. Part way through his second year the teachers and I started noticing that he was not as mature. Academically he was doing great! He was just not as flexible and got upset easily. I can't remember everything, but it became very clear by the end of the year that he was not as mature as his classmates. Socially, he just didn't belong with this peer group.
We were so fortunate that our school district offered a 2 year kindergarten program to kids who qualified (they did screenings). Our son did the 2 year program and will be in 2nd grade this fall. All of his teachers have agreed that he fits in very well with his peers. He could be going in to 3rd grade this year, but he is doing so well where he is.
Sometimes letting them begin kindergarten at age 6 is the right thing to do. It's crazy to send him just because he made the cutoff. If you think he would have a better experience if you wait a year, then wait a year. If you think he's ready, send him. But don't send him just because he's old enough. It has to be what's right for him.
I cringe when people try to warn you not to send him when he's 6 because then he will be 18 when he's a senior. I have a spring birthday, so I was one of the last in my class to turn 16, get a driver's license, etc. It was the oldest in the class who were always the coolest and the most mature. And while I understand that at 18 they are adults, if you instill good values and a value for education, what difference does it make? When I was in high school, my eyes were always looking towards college. What did I want to do, what was I going to major in, where did I want to go? My brother and sister and I knew that college was a requirement in our family. Not in an obligation kind of way. It was just what you did next, and there was never a question of whether we were going or not. The only questions were where to go and what to major in.
Being the oldest in the class can only be an asset, especially for a boy. Many really do need that extra time to mature. If he is ready, great. If he's not ready socially, give him more time.
I talked to many, many parents before making a decision. Of those who sent them at age 5, several were happy with their choice, but many say looking back they wish they had waited. They said middle school was particularly hard as the older kids were going through changes before their own child, who then had trouble fitting in.
Of those who waited to send their child, almost all said they were so glad they made that decision.
I have yet to hear a parent say, "I wish we had sent him sooner."