I am writing as an adult who was accelerated twice. My experience is that the social and developmental issues are very real, and have effects for decades into a child's future. We don't usually know who will be a Thomas Edison or an Albert Einstein, but in general it is helpful to a young person to have the ability to manage their own environment and to have adequate social skills vis-a-via their particular family and social situation. We have to take a careful look at what constitutes a successful adult life, and be sure that our children have the opportunity to grow in those [soft] skills through their day-to-day experiences.
Middle school presents a lot of challenges regarding organization and self-reliance. Maturity (years) usually helps.
Giving a child the chance to have interact with age-mate peers who are on an advanced level in one or two areas, perhaps on the outside (specialized vacation camps, local library or arts center) can meet the need for intellectual stimulation and like-minded friends.
The struggle to socially and developmentally keep up with older classmates eventually erodes the healthy kind of self-esteem that should lead to happy adulthood. It is hard to think about it now, but you don't want you kids to get involved with a disenfranchised crowd.
NYC native