my oldest just started kindergarten. he's birthday is in january & will be turning 6. he obviously couldn't have started last year, & honestly, I felt that he would have been too old to hold out. he would have been 6 turning 7.
My middle child on the other hand, I already plan on holding him back. With a birthday in late October, I just think he will benefit from that added year.
Personally, I think the boys are usually the ones to benefit most from being held back, but that's just my own opinion.