My parents had 2 children born less than a week before the cutoff date (myself born dec 29th when the cutoff was dec 31st, and my sister born Aug 26th when the cutoff was moved to Aug 31st). They sent me to kindergarten at 4, and my sister was held back a year.
I can honestly tell you that sending me to K at 4 was a horrible decision. I could read at 3 and there was no pre-school option at the time, which is why my parents sent me to private school. Academically I was among the top of my class, but it was a constant social struggle for me.
I had the same problems SRUAlmn had. Only to make it worse I was the youngest in the class (the other Dec kids were all held back before K except for 1 other girl), I was the tallest in the class, and I still developed 2yrs earlier than everyone else did. Try being a year younger, almost a foot taller than your male classmates, and being a D cup when you're 13. Not a pleasant social experience for an all ready introverted child and the horrible body image is still with me. and I also have problems applying myself because of the ease in which I found classes to be. I especially have difficulty applying myself in subjects I find boring and struggle with (math).
My sister, born 10yrs after me, is almost the complete opposite. She had seperation anxiety so my parents decided to hold her in pre-school another year. She graduated 2mos before her 19th birthday. She was a social success, an academic success, and an athlete. Most of her friends, and her high school boyfriend, were/are almost a year younger than she is but it never affected her at all. For her, being held back was the right decision. But don't get me wrong, she also has problems applying herself because she never had to work at her studies