I held my DD back a year from starting Kindergarten.
She could have started this year being 3 turning 4 in March.
I decided to hold her back so she would be 4 turning 5 and then 5 turning 6 for prep.

I've found with my DD in the last 6 months since she turned 4 that she has developed so much more socially, emotionally and has a lot more of the skills fine tuned that dachlostar posted.

I also took into consideration that if I had have started her at 3 turning 4 by the time she got to yr 12 all the kids would be 17 turning 18 and she would be 16 turning 17. She'd want to be going out etc with her friends but still be underage when some of her friends might not be.