If a child has only skipped one grade, the age difference is less of a big deal. When the gap gets to be two years or more, differences become more important as kids enter adolescence. They aren't as important before that.
I know that gifted kids have cognitive abilities that equal or exceed those of kids who are older than they are. But when you have an pre-pubescent child mixing with kids who are 2 or 3 years into that process, the physical differences are real. There some ways in which the kids of those two ages won't connect.
