we "redshirted" our oldest, for a variety of reasons - (preschool recommendation by 1 of 2 teachers, speech, size and the looming first year of all-day K.) The pros: He was old, confident, tall & big in Middle school. The Cons: having to always explain why he's a year older than many classmates, somewhat bored at school. We could have gone either way, but the small boost in the TOUGH middle school years was worthwhile for him.
we sent kid #4 to school early. she was 4 for a full month (late sept bday). she was ready for school, 7 years later she is top of her class, smart as a whip, working hard. The pros: she's been challenged and done well. the cons: She's in MS now. she's smaller, socially and physically not as mature as peers, and that is hard on her.
In HS it will even out as we've seen with our other kids.
Kids seem to even out in both elementary & HS; but middle school is hard on kids no matter what. Kids' height, weight, clothing, hair, skin, athleticism, socialness, likeability, economics (and sometimes but not often academics) --- those are all part of the mix in Middle School. Even with all of the anti-bullying, diversity messages & "no place for hate" messages -- i still see it: the smaller boys get picked on, the bigger girls too; the socially awkward kids as well.
RIGHT NOW, in 6th grade, i often wish we had waited with #4; just because an older kid's mental maturity is more developed to handle the situations that i see everyday at her school (make up, body developing, texting pictures to boys, body image, Sports, etc. etc.).
In a few years i probably wont have those regrets because these things all change. We do our best and our hardest to support her at home as she keeps growing and maturing. Very excited about the trampoline we just ordered to get her off her phone, and outside!
** and i do have to say, a smart kid will be a smart kid independent of starting early or late; it will not make a difference with scholarships later on.