This has nothing to do with the school you choose, but you might want to consider what they are telling you if your son is not extra large in size. These days, everyone is holding their kids back, and half the kindergarteners are 6, some close to 7. Especially the boys. At first, people hold their sons back for maturity. Many boys are not able to do all that sitting and fine motor stuff at 5. (Don't get me started on how horrible it is that K is now all academic, no blocks or toys in the rooms!)
Then, the other moms of even mature, bright boys hold their sons back so theirs won't be the youngest and smallest, something boys hate (for some reason, LOL). Therefore you need to adjust YOUR child's age at the different grades so his class is filled with developmental peers.
From a mom of 3 boys -- just thought you ought to know. Girls do not care so much if they are a bit younger or smaller. For girls, if they are mature and can relate, their status comes from other things. Boys do not like to be younger or smaller than their peers.
That depends where you live. In California in the South Bay area (San Jose), most of the kids start as young as they can.
We didn't hold our son back. He's in 7th grade, his birthday is on Wednesday, and he'll turn 12. He was one of the smallest, but now he's not. He's started puberty earlier than some of his friends and now he's the youngest and one of the tallest of his friends.
My son is also gifted and does very well academically. He's currently taking 9th grade math. He is very happy that we did not hold him back.
Girls do not care so much if they are a bit younger or smaller
^^^^^^^^^^^^^^^^^^^^^^^^
I was the youngest child ( a girl) in my class. I was also late to mature.
It was hell on earth.