The school apparently considering moving me up a year in primary school, but in discussion with my parents, decided not to, for social reasons - I was very, very shy.
I don't know if it was the right decision or not. I was one of the oldest in my year, and found it hard to have normal social relationships - partly, of course, because of the shyness, bu also partly because of being academically ahead in many ways - I was seen as a sort of 'older sibling' I think. I often had the job of helping in the classroom, tutoring, marking, preparing displays, all kinds of things like that. I'd read stuff for people when they couldn't read, etc. And it wasn't really an equal relationship, not proper friendships somehow. I wasn't disliked, but at the same time, didn't really have friends with the normal give and take aspects of friendship, or with the same interests (I'd have rather been reading, and not what the others were reading). I wasn't challenged academically, and as a result, became a perfectionist in what I did do, rather than learning to push myself. There was a huge amount of pressure to be seen to be perfect, because of being top of the class in many things, and that was really hard - and again, made me somewhat separate from the others.
There are certainly ways that I think being in an older class would have been good socially - I'd have had more of the equal peer-type relationships, rather than older sister/younger sister ones, always looking after or helping role, which was very one-sided. I'd have not stood out quite so much perhaps, which might have helped. And might have learned to challenge myself despite the potential for failure, rather than end up doing well so easily that I became terrified of being anything less than perfect.
In other ways, it would have been difficult. I was old for my year and an early developer physically, but not mentally - was scared of growing up, although I was mature/wise/sensible, but didn't like the idea of boys, teenagers, etc. So being in a year ahead would have meant I felt even more out on a limb in that sense - though I don't know if it would have mattered as I didn't end up going out with anyone etc for a good 15 years after the norm anyway!
But just pointing out that it's not as simple as saying that an academically ahead child is better off in their own year group as that's where they will fit socially - sometimes you don't fit socially simply because of being ahead, and just end up being treated differently as a result - not badly, but just having different relationships instead of normal friendships, and you can miss out as a result that way as well.