I don't know if you caught this story last night on 60 minutes:
Redshirting: Holding Kids Back From Kindergarten
I have heard, read and seen convincing arguments for both sides of this debate. In my opinion, I think parents know what's best. You are with your child the most and know if they are socially and academically ready. There is not a researcher or author that knows your child better than you! I'm interested to know your thoughts..email me if you want to discuss :)