I see exactly what Howler is seeing in my area. I do believe these parents feel they are helping their children. The ones that are really annoying will be in any venue. They are the 1-uppers in life and I truly try to avoid them.
Having children from 6 to 20 years old, I have seen this play out over the years. In general, they don't seem to get the results that lead to their decision to redshirt. We all have an aptitude limit. They falsely believe holding their children back is going to change that fact. Truly average is not going to become gifted by redshirting. In my area, these same parents are helicoptering over every moment of their children's lives. I know they believe they are helping, I just don't. I don't think they are teaching their kids how to achieve on their own or how to take care of themselves later as adults. It's a general theme in the environment and I think hurts the kids long term. The real impact is not on my children. It's on their children. The only issue I had was initially it took the school a while to see I wasn't one of them.
My district uses CogAT, Olsat, and State achievement testing all grade normed. I know CogAT and Olsat offer age norming, but my district specifically uses grade normed results. My dd6 has 9 year olds in her 2nd grade & my DS13 has kids turning 16 as freshman. It's not the kids right on the line of cut offs. I don't know in my mind that I see that as redshirting. These kids are 18 months to 2 years past the cutoff. At the end of high school, I am not seeing these kids excel. Given the numbers in my school, these are not all kids that had some other issue going on.
Ultimately, the parents aside, I feel for these kids. They have so much pressure on them from the parents and they are being taught that being #1 comes first and foremost. The parents believe you gotta be the best. Our circle of friends are the goofy lets go splash in the mud type of parents. I am thankful I have found sanity amount the parents I have befriend. Parents that understand that raising gifted children is not as easy as some believe. I never even thought about my kids being gifted when I had them. I just hoped for them to be healthy and not have to many struggles or obstacles in their lives. Gifted has been challenging and it's topped off with a variety of 2E and/or health issues with each one of my kids. I don't think these parents really want to be in my shoes if they understood it.
Here interesting little article about why it may not be such a good idea
http://www.nytimes.com/2011/09/25/opinion/sunday/dont-delay-your-kindergartners-start.html?_r=0
