I attended NYC public schools, and I was always the very youngest in my class. My parents never considered red-shirting. I didn't mind being the youngest. I did mind being the smallest, as it always put me at the front of the girl's line when we traveled outside of the classroom (I was painfully shy, and you can't hide when you are always at the front of the line).

My son has attended NYC public schools, and has always been the youngest in his class. He started kindergarten at PS 29 when he was 4 1/2. He was academically and socially mature, but he was overly sensitive to noise and commotion, and he didn't like switching activities as often as the teacher did. The adjustment was difficult, but we got through. Despite his relative age, he has always been taller than most of his classmates. He is now a senior in HS, and has classmates some who are 2+ years older than he is. 

When I was a teen, being older than classmates meant adult privileges, such as a drivers' licence and the ability to drink legally (yeah, I'm that old). By contrast, my son has an urban social life (no one drives) and the legal age to drink has been moved to the other side of college.

As a teen, I was consciously thankful that I would "get past high school" as young as possible. Over the years, I have asked my son if he ever wished that we had started a year later. He has always said no.