I like to offer the non-academic future perspective. My daughter is an Aug b'day so she is the youngest in her class and she is now 10 and in 5th grade (will turn 11 next Aug). Academically she is ahead of the game, and she is tall for her age so she doesn't look out of place.
But let me offer this perspective. She is in classes with other kids who WERE held back, so there is QUITE an age gap in the class.
When she was in the 2nd grade (and 7) there were 'older girls' who were already 8 and turning 9 and one girl started her period.
Now she is 10 and there are girls who are 11 and turning 12 in the next couple of months in her 5th grade class - they all have their period except her. Some of them have kissed boys. They text and email COMPLETELY crazy messages. She feels a bit alienated, cuz she's not at their level.
Yes - go get friends with whom you have much in common. We have those talks. She does. But it's REALLY hard to feel extra out of place at a time when feeling in place is so very important to kids.
She will be the last kid to drive (yes, I consider that a blessing - I may not let her drive til she's 30). But every other kid in her class will be able to drive and she won't.
She will have friends who are 'dating' seriously a year or two before she is their same age, but she will be in that social circle because they are her classmates.
She will be 17 when she goes to college and will turn 18 after the 1st week of college.
I was a big proponent of go ahead and send her cuz she's smart and tall and socially ready and could follow all the directions (day care baby) etc. Now, I think I would have held her back. I have had to have "conversations" a year or two earlier than I really wanted because her 'classmates' were experiencing things that I wanted to make sure she could handle.
Good luck with whatever you decide. It's a tough decision.