I went to a multi-age/grade Montessori until 4th grade. After that, public school where I was one of the youngest... no biggie.
Another girl in my grade who was almost 2 years younger than the cutoff (she was from Scotland, and arrived in the US starting in 4th grade, too). And yet another who was a full year younger--as it turned out, she actually was brilliant, but her parents didn't want her skipping all sorts of grades to go to college very early--they wanted her to have friends, etc.
Note all the really young ones I mentioned were girls.
My father was held back a year back in the dark ages for the stupid reason that he had poor spelling (he was home schooled until 3rd grade). After that, he said he was more mature and more serious "about life" than other kids in his grade, even in college--though the reason for this might be a combo of his temperament and the fact that he had some pretty major family and work responsibilities starting at a young age.