Hi M. C,
I'd like to share my perspective. My daughter has a LATE b'day. She is at the mid-Aug and the cutoff here is Sept 1. She is pretty academically advanced (plus she's tall) so she fits in GREAT with her peer group. There is one other girl in her class with the exact same b'day as her, and they are the youngest 2 in the class.
I thought this was not a big deal. She was extremely verbal as well as socially and academically on par with these kids.
However - there are kids in her class that are a full year and 1/2 OLDER than her. Kids who were held back, kids who have a Sept b'day after the cutoff etc. So immediately in Kindergarten she was 5 with kids who a month later started turning 6.
K/1st is not that big of a jump. But fast forward 2 years from now through the rest of high school.
In her 1st grade year, she had a split class due to enrollment. So when she had just turned 6 she was in 1st/2nd grade class with boys and girls who ranged from about to turn 7 to HAD ALREADY TURNED 8 when the school year started (for the 2nd graders). By spring some of these kids were having their 9th b'day. In a classroom with my 6 year old. Who could do all the assignments and was getting straight A's and making friends.
A 2 year span might not seem big.... but it is. The shows the older kids watched were different from what I wanted my 6 year old to hear about (ie Degrassi, Malcolm in the Middle, Oh and the movies.... Yikes.) I didn't even want her to HEAR about some of these topics in daily conversation. So I had to have "what does it mean to cheat on your boyfriend" and "Why can't I see xyz movie (PG13 movies)", and "When can I have a bra" and "why did Marie have her period so bad that she had blood all over and her mom had to come and give her new pants" conversations MUCH earlier than I had planned. I didn't want to have these conversations at 7.
But she was 7 in a 9 year old's world.
I ended up changing her school (for an unrelated issue) but it did get me thinking that she would have this peer group when she was 13 and they were in drivers ed about to get their drivers licenses. She about to turn 11 now - and I talk to mom's whose kids were in that class and some of these girls are dating and wearing makeup!!!!!!
It's not always about is your son ready socially. It's do you want him to have the experiences and frame of reference that someone older than him has naturally at the right time? You'll have to have conversations with him about kissing girls, and condoms, and why some of friends boy parts are doing stuff....... a year or 2 earlier than you otherwise would have to have these conversations.
Coincidentally, the school my daughter attends now - her class is the oldest class. She is still the youngest in that class, but there aren't kids 2 years older with whom she interacts daily. I prefer it. I give her academic support outside of the classroom and her school is based on an individualized curriculum, so she can go as fast and as deep into ACADEMICS as she is able.... without the pressures of going fast and deep into the teenage world when she's 11.
Good Luck