We skipped DS7 into Kindergarten (his birthday is at the end of November). He was the smallest kid in class even when he was with kids "his age".
He's now in a 2/3 class (should be in 1st based on birthdate) and is still bored. Will we skip him again? Most likely not. However, I am glad we did the original skip because he would have been AWFULLY bored in class if he was in 1st grade now.
Granted, I don't know what it's going to be like for him as a middle/high schooler, but I think it would do more harm than good for him.
I was the youngest in my grade and I never felt "behind" when everyone else was driving, going through puberty, dating, etc.
I think there were some great points made for both arguments. I hope whatever you decide is good for your family and your children.