I tell my kids to try not to talk about their ages until the other kids get to know them. That way their ages aren't seen as primary thing defining them.
My daughter has been 7 for about two weeks and is in 3rd grade. Even so, she's slightly taller than most of the kids and fits in perfectly. For her, second grade would be the place where she'd stand out like crazy. She'd be a head taller and light years ahead.
My son skipped two grades and so is smaller than most of the kids in his class. But he has good social skills and fits in pretty well. No one has asked him about his age yet, and the school has been wonderful about accepting him. There were no questions or issues whatsoever about his age. Small brag: the kids had to take a math test, and the top 15 scorers were picked for a group that will train for the Math Olympiad. For some reason, they were given the same test that they took last year. DS (who didn't go to that school last year) scored 4th or 5th, in spite of the fact that he was competing with kids who had already taken that test once. I was proud of him. The school is GT friendly, which is really nice.
Fortunately, no comments from parents yet.