J.S.
I can only speak from personal experience, but I am a Dec. 31 baby. I went to kindergarten at a private school (Montessori) when I was 4 1/2. After that I went to 1st grade at public school - my b-day was past the cut-off date, but my school let me in because I had already completed kindergarten - they didn't make me repeat kindergarten just because of my age.
My son now also attends Montessori. I know that his particular school is pretty lax about the age thing - they test the kids before enrollment and admit kids based on how they do on the tests more than age. I mean, they're not going to put a 3 year old in kindergarten, but if a kid is just a month or two past the cutoff, they don't care as long as the kid is bright, both academically and socially.