In my district, you need to be 5 by Sept 1 to start Kindergarten. My DD is a June birthday and I actually held her back a year due to a sever speech delay (and she totally understands that is why as she just told someone that last week) and she also has ADHD that we were just getting diagnosed and she would not have been able to handle kindergarten at the that time. When she did start kindergarten this fall, she was so ready and has done great.
I do know of 2 people that have DD's with Sept birthdays who tested them and they both started kindergarten a year ahead. Both are now in 1st grade and seem to be doing really well.
I also know 2 of my friends who had their kids start kindergarten on time (one is a July birthday and the other is a fall birthday in a state with a Dec cut off) and both boys are now repeating kindergarten as they just were not ready - both had social issues and not educational. One of the boys gets pulled up to 1st grade for math so they can keep him challenged but he is with the kindergarten's the rest of the day.
I have another friend who started her son (Nov birthday in a Dec cut off state) on time. The school recommended that he repeat kindergarten as they felt he was just not where he should be but his parents refused and he is now in 1st grade and is still struggling to keep up.
I think when you start your child is 100% up to you, their is not a right or wrong and it is very dependant on your child. IS your child ready socially and not just academically? How will your child do being the oldest or youngest in class now and also at 13 or 16 or when they go off to college?
Being ready for school is about a lot more than just reading and writing and math but being able to take care of themself, make friends, sharing. Can your child put on his boots and coat himself, can he handle all the bathroom things himself? Can he behave on the school bus?
For me, holding my DD back was the biggest gift I could have given her. For a friend who has a son 2 weeks younger than my DD - - holding him back would have hurt him in the same ways it helped my DD.
Every child and every situation is different. I personally would never test my child to start early - not for how she would do today but how she would do as a teen and as a young adult going off to college. I just don't see the point in rushing it. There are many programs out there for those kids the year before kindergarten - the PreK my kids are in (went to) is amazing and treats the kids more and more like they are in kindergarten towards the end of the year to get them ready.
A little girl in my DD's PreK class is an Aug 30th birthday so misses the deadline by 2 days - this poor little girl has struggled all year to just keep up with the other kids, some who are are almost a full year older than her or even more if they had been held back. She (we know the family from daycare) is struggling in all areas including making friends and all the academic areas. The family has already decided to hold her back from starting kindergarten in the fall and will have her repeat the PreK class.