I also think at kindergarten age, you are actually just separating by age. A kid with an early birthday could have lived 11 months longer than a kid with a late

I also think at kindergarten age, you are actually just separating by age. A kid with an early birthday could have lived 11 months longer than a kid with a late birthday, and when you are only 4 or 5, that's a good chunk of your life. See Malcolm Gladwell.