DS7 (PG), is a late October birthday with a Sept. 1st school cutoff. Like someone said, all subjects are still way too easy for him even being accelerated a year, so it is not helping much. The important thing is if the school is willing to subject accelerate. Ours is which is the only way it is working for him in school.
BUT , another thing you might want to think about is that highly gifted children usually have a more advanced way of thinking about everything. My son complains that he feels so much older than the kids in his class even though he is a good year younger than a lot of them. There is not much mental interaction with his friends, he does like the playing on the playground though. But this is pretty much the only thing he has in common with the kids in class. I can't even imagine what it would be like if you were in the class he is supposed to be in.
Your older son is highly gifted, chances are you're younger will be in that range as well. I would definitely go ahead and start him and not hold back. It will be easy to keep up with the academics and he will probably feel more like he is with kids who are somewhat peers to him.
We have done the same thing with our 5 year old son. He is even younger, a late November birthday. Academics, especially math is still way too easy. And I can tell that the maturity level is not a match either. He is in a young class though. About four of the children are May /June birthdays. I really don't think it is going to be a good match for your son if you hold him back. Not on any level.
