My oldest son's birthday is September 25th. In my opinion, it depends on the child. Being the oldest and even going to preschool for 2 years, he was bright academically but socially immature, very cautious, had some trouble with his writing and coloring neatly. The teacher suggested we hold him out and put him in the following year when he was 5 going on 6. We agreed and I think it has been best for him.
I have heard others whose children had birthdays in September and October wish they had kept their children out. One woman I know spent the first month of first grade crying because she realized her son was not ready when he was in kindergarten. the school wouldn't hold him back because "they don't like to do that". (Personally I think they SHOULD be held back in K or 1st vs. later like in 5th.) So in first grade, he was constantly being taken out for tutoring which was affecting him negatively...kids knew why he was being pulled out, it was embarrassing to him, and he was missing out on learning how a classroom works and working in groups.
If your son is academically AND socially ahead, then perhaps he is ready. If not, then I would wait. Keep in mind too, you are going to be putting him in preschool. Those teachers will be able to advise you as he gets closer to school age.
Now having said that, my middle son's birthday is the end of July. I wanted to keep him back too honestly because while he was socially ready, he wasn't academically ready and I didn't like he was going to be on the young side. We decided to put him in the year he turned 5 and I have to say, he needed a little extra help in the beginning but now he is doing amazing. It was what was better for him. It's one of those things where I think you know your son best and that's what you have to base it on.
Most preschool teachers I know suggest holding back though and also say it's about the first 5 years in school and then their Jr. and Sr. years of high school that you really see the benefit of holding them back. I am hoping putting my middle son in at 5 doesn't come back to bite us but I have no concerns about my oldest because we waited.
Good luck!
L.