On the other side of the coin you have my neice. She started K at the age of 4. Her September birthday was a week before the school district's cutoff date. She began college at the age of 17 and was unable to sign for her own student loans because she was not legally an adult yet. She started her senior year at the age of 16 and was never able to drive to school. She will be the last of her classmates to turn 21 and has had to sit out when all of them planned casino trips to celebrate that milestone birthday. She was ready to start K both academically and emotionally so I don't think that my sister even considered holding her back for a year. But being "young" has always been a sore spot for her.
I was able to get my son's schedule when he started college because he was not 18 yet. The woman was not going to get it for me (he needed it and was working) but then saw that he was still underage so she gave it to me. She said she never sees 17 year olds start college. No because kids are graduating at almost 20 from high school. I cannot see how this helps kids in the long run. By 18 you should really be a senior, not a junior and in some instances, a sophmore.
My son never cared about being the youngest. He is a big kid and looks a lot older.