I think I agree that the maturity of Kindergarten has changed and made it more difficult for children who are at the right age. If a high percentage of the population holds their children back due to maturity concerns, then Kindergarten might as well have a 6 year old entrance age. I think it also sets unreasonable expectations of the maturity and energy level of children in Kindergarten especially boys as another person who replied said. We already have a school system that lets boys down more than girls...it worries me to perpetuate yet more ways that boys are at a disadvantage.
I see this issue with my son, who is right at age level. He is younger than many children in his class, so he is seen as not acting his age. At the same time, holding him back would have caused bigger problems. Both he and his brother need intellectual stimulation, and their behavior in pre-k was getting worse due to lack of good material.
