I was placed with agemates all through grade school. I felt like a freak, my self-esteem was terrible. When we moved, I had an opportunity to skip 7th grade and attend a GT school. I was afraid and reluctant to do it, but my parents had me test anyway.
I was accepted and attended that school for three years until we had to move again. It was the best thing that happened to me until college
After our second move, I went to a regular public highschool. There were a few snafus with getting proper class placements, but I was only there for two years.
My DS was accelerated from PreK to K (private school) when he turned 4. It didn't work out well because the teacher sabotaged the skip. He went to K again at the public school and was accelerated to 1st in the spring with excellent results. He is now happily attending 2nd grade.
From my experience, I would say that it is crucial to consider the circumstances that the child will be skipping into. What is that group of kids like? What are the teachers' attitudes? What will the curriculum be like? The initial months of a skip are an important adjustment period. If all goes well, the child will feel successful in his classwork, and accepted by the teacher and other students.