My oldest dd is also on the opposite side. July birthday (here, cutoff is 9/30 but slated to possibly change to 7/31, rumor has it.) She started school right on time, Just-turned 5 so the second-youngest in her grade at her school. Several years in counseling, testing, psychoeducational couseling, and trying creative solutions at school, she was moved up a grade provisionally and then permanently. So now she is in 8th grade at 12, an Honor Roll student, with more friends than she ever had before (she used to not enjoy time with other students her age, because they never understood her or her sense of humor). She knows that she will not be driving till senior year of high school. She knows that she might have to go to a college nearby instead of living in a dorm. She knows that she cannot do all the things kids in her grade do, because it is not all appropriate for 12 yr olds. But has never regretted moving up, so I don't regret it either. We did what we had to at the time, after carefully exploring options. If, someday, this is no longer the best place for her, we will once again explore options. As for sports, she will be in the largest 3-grade (10th-12th) high school in the state, where the sports programs are tough to get into since so many kids try out. So she opted to participate OUTSIDE of school, with kids her own age.
My nephew, on the other hand, August birthday, only child, destructive and immature. Did not know how to play with others, or be responsible with others' belongings, etc. He was held back, started K at barely 6. He is still a bit socially delayed, according to his teachers, but that just reaffirms that for him, starting at 5 he would have been MASSIVELY delayed socially.