I am not for or against sending kids early. Every child is different and therefore the circumstances/outcome may be different. We totally agonized over sending my (appropriately aged/late June birthday) daughter to Kindergarten after her preschool teacher strongly suggested we first do a Jr.Kindergarten. The teacher felt she needed another year to play. I finally had her tested (Gezelle)and she tested at or above her age all across the board. We also looked at the kids she would be attending with in both scenarios and felt that if we held her back it may not be a good thing for her socially/emotionally (there were a handful of girls in that grade who could be downright mean). There was a lot to think about, but we ended up sending her to Kindergarten instead of holding her back.
So....when she started, she was indeed better off with the group of kids in that Kindergarten class. And that was a great thing for her confidence and self-esteem. However...she had a VERY tough time transitioning to a school day that was 85% schoolwork and only about 15% "play". And that was just a half day of school! She used to constantly tell us how she just wanted to play. Her teacher truly knew she needed to play for another year. She is now almost 7 years old and will be going into the Second grade in the Fall. While she finished first grade above hgrade level in reading, she struggled to get to the class averasge in math (but did finally get there). I am nervous for her for Second grade because my son just finished and so I know firsthand intense the curriculum is for Second Grade in this day/age. We don't regret our decision, because we have seen a lot of issues unfold in the class behind her with those meaner girls---and a child's confidence and self-esteem is more important than academics in our book.
but all that being said, here are some things to think about...
Although your son seems ready, more and more children are "waiting" a year. Therefore, there MAY be quite a difference in where the class average ends up falling.
Today's curriculum---even for Kindergarten---is almost criminal it is so advanced. They now have homework in Kindergarten (typically once a week but every night in First).
The children he will be going through Kindergarten with are the ones he will go through school with for the rest of his schooling (unless you move of course). Keep that in mind.
In your situation, you may/may not want to hold him back if his younger sibling will end up in the same grade with him???
Remember that school today is real work and it is the official end of sweet carefree playtime.
Most importantly, follow your gut instinct, no matter how slight it might be. To this day, although my daughter struggles a bit with math, she is a really happy girl who likes to go to school every day...and I'm not sure that would have happened had I held her back.
Hope I gave you some good things to think about and didn't confuse you even more. It was shocking to experience how agonizing that decision was (it was even worse than deciding between public vs. private!). Good luck with your decision F.---it is a confusing one!