My DS was 8 in 5th grade this year, but he started kindergarten a year early and then moved up again from 4th to 5th. He's in a small school and was already really close friends with most of the 5th graders, so the bump up was good for him both academically and socially. He's also very tall and mature for his age, so he barely stands out. The biggest challenge has been with organizational skills (bringing the right books home, doing the right assignment), but the school has been very flexible and willing to work with him.