So, here's my 2 cents.
I have a June birthday, and started on time and within my state's cutoff. I was always one of the youngest in the class, and did well academically and socially for the most part. I could have benefited greatly by being redshirted. 17 was, for me, too young to finish high school. I struggled with math after 1st grade and had to have a lot of tutoring just to keep up. My English scores soared, but that was because I was naturally predisposed to excel in reading and comprehension (not bragging, I just did well). If I had a child, boy or girl, with a summer birthday, I would base my decision on that child, and not the fairness to the rest of the children in the class. I would only redshirt if I truly felt that the child needed the extra year to mature, not just for an academic or sports edge. My brother and nephew both started on time, and both had to be held back to mature a bit.
Now, my husband has a March birthday and was redshirted. He turned 7 in Kindergarten. He was also our valedictorian. Yes, we graduated together, and he's a full two years older than me. His father (our high school principal) is of the opinion that no children should start kindergarten before turning 6 because they need the extra year to learn basics before being thrust into academia, and the older children, in his opinion, make better decisions upon graduation.
I neither agree nor disagree with his opinion, however I do take it into consideration as we are making the decision about when to enroll our DD4 in kindergarten. She also has a March birthday, and if redshirted would turn 7 during kindergarten. My husband is determined to hold her out because of his experience, and I'm not sure that's the right decision. I think that, although she's too young now at 4 (and well behind the cutoff anyway, so we're in keeping with the school district's rules) next year she would probably be fine. My husband has agreed to have her evaluated as we get closer to enrollment, and take into consideration whatever comes of that. I'm confident that she'll have the skills to enter kindergarten on time, so I imagine that's what will happen.
I think children who are like me, and could benefit from an extra year before starting kindergarten should be given that chance. If they truly need the extra year to mature, then it won't matter if they turn 7 during the year, because they'll still be on par with the rest of the students. I certainly wouldn't appreciate the school district stepping in and telling me that my child who's birthday was a week before the cutoff was just too old. That's silly.
Oh, and frankly, I'm of the opinion that 4 is too young to start kindergarten. You may not agree with me, and I'm cool with that, but it's my opinion and I get one.
One last thing, I hit puberty rather earlier than many of my peers, and as I mentioned, I was one of the youngest. I was teased about it (although I survived, thank you, and those same boys were singing a different tune a few years later). Anyway, had I been held back, that could have been more of an issue. It's one thing to have to start wearing a bra in the 4th grade, it's quite another in the 3rd. So, as you see I'm still conflicted on this issue, even in my own case.
ETA: It should be noted that I don't contribute my husband's success in academics solely to his being a year older. I'm sure it didn't hurt, but he's a smart guy, and he's got this amazing work ethic that had a lot to do with it. While we were out at the movies or the burger joint, he was home studying. That's a big part of why he ran circles around the rest of us.