I would consider the personality of your child in making your decision. My DD was very intimidated by the 7th/8th graders when she started school this year, and definitely appreciated the separation. At her school there seems to be a culture of "we had to be the bottom of the heap last year and you better bet we're going to make sure someone else is in that role this year!". Nothing drastic, but definitely the sixth graders are considered the babies of the school for a good chunk of the year. It took awhile for DD and other sixth graders to develop a "whatever" attitude and stop caring what the 7th and 8th graders acted like.
Just from looking at the way kids dress, there is a culture shift that happens over the course of the 6th year that would have been overwhelming at the beginning of the year. I like that she had time to adjust to that more slowly. As to whether the hormonal gap impacts gifties the same way....hard to say. In our case, I would say DD has been on the slow side with the hormonal shifts (also the young side, with a late summer birthday) but on the advanced side in her ability to notice and assess the way in which others act, form their groups, and treat other people. This has been a blessing and a curse. Her ability to see it for what it is makes it not at all appealing to her, so I guess it's good that she was largely among sixth graders where not everyone has crossed over. On the other hand, she is hyper aware of the unwritten rules and that she falls outside of what is considered "cool". If she were less aware she might try to fit in. As it is, I think that she is not, and probably never will be, able to act in a way that she finds at best, kind of silly, and at worst, mean and destructive.
OTH, DD was in a 7th grade math class, and while she felt a bit shy about it, no problems in the classroom at all.
She was bored to tears in her language arts until she was able to do something outside of the classroom, but has found social studies and science to be a good fit.
I guess the questions I would ask myself are:
How does your DS feel around older kids, and is he still on the naive side, or has he already begun to cross into the teen attitude?
How do you feel about him starting highschool earlier? This may depend on where you live and what kinds of issues kids are likely to be exposed to at his particular high school.
Do you think that there is little to be offered by the curriculum in all academic areas, or are there a couple of areas that you think would be appropriate? If so, can he subject accelerate in just the other areas?
Is it a now or never attitude with the school? Can he skip up mid-year or skip from 6th to 8th at the end of this year?
As with everything, there will be pros and cons to both.
