I've never even heard of half day Kindergarten. Even the church preschools that offer 5K are full day programs here (in SC). We have a September 1st cut off for public and private schools. We live in an affluent area and red shirting is rampant. My oldest is in a classical university model school and a high percentage of the kids there are red shirted (I fail to see the logic in holding kids back, putting them in a grade a year behind, giving them "advanced" work and calling it rigorous). The youngest child in her class has a birthday in January.
My 2nd child is in public kindergarten - same hours as the rest of the school 8-2:30. No nap. One recess at the end of the day. The age range in is class is 18 months! The oldest turned 7 in January (so he 6 1/2 when he started) and the youngest started the year at 4 years old in August and turned 5 on September 1st. Several of the summer birthday kids were "lapped" by red shirts. His class of 24 kids has 7 redshirts. Not just summer birthdays - January, March, April, several Mays. I can't imagine as a teacher having a class with that big of an age range. Many of the parents of younger kids feel like they have to hold their kids back because so many of the spring birthdays are now held back. Who wants their daughter in a class with boys 18 months older in middle school and high school?
I also have a child at a private 4K (he goes 3 mornings a week but all the other kids in his class go 5 days). The teacher recommends many of the children are held back. She scares the heck out of the parents of these kids about Kindergarten. They don't want their kids to fail or have a bad experience that will scar them for their academic life so they send them to private church preschool 5K then on to public school for another year of Kindergarten. That is what most people do around here - two years of Kindergarten at two different schools.
We also have a county wide gifted school based on test scores in 2nd grade. These are grade norm so the younger kids are at a huge disadvantage for entry to this competitive school
I think it is out of control. Most of the parents talk about wanting their child to be confident and not being small compared to other kids. Sports play a big role too. I hear about not wanting them to be last to drive etc. My nephews were both held back (summer birthdays). My sister in law said she loved how when my nephew went to Kindergarten (for the 2nd time) he knew everything and was so confident. They bragged that he was one of two kids in his class that could read well. She loved that he didn't have to try very hard that second time around and he could enjoy himself without having to worry about learning much.
I don't get it. My youngest son has an early May birthday and I get asked all.the.time. if I am holding him back. No way! He's a bright kid! He won't be reading as well as my other son (Oct. birthday) but he will be on his own trajectory. And that is OK. All of the research I've read says the advantage peeters out by 3rd grade then they are just older and more likely to be bored when they are 19 in high school and living at home.
If my kids take a gap year it is going to be when they are 18. Not 5.