It has been really helpful for DS (also DYS) to attend gifted weekend workshops held at a major university. There, he's met other kids who are also in "asynchronous" placements or homeschooled.
He is currently with age mates (although he's on the young end)for about half the school day and accelerated two years for three class periods. This is largely his choice (to stay with the split vs. whole grade acceleration). Fortunately, the older students are used to him and even applauded recently when he was the one to answer a tough question correctly (the teacher told us, he didn't).
As for explaining it, his frustration levels were so high when at age-based grade level, he was glad to move up (if maybe a bit nervous at first). We have talked with him the drawback of the current traditional system being age-based placements instead of those which let children move through at the right pace for them. (This was a fascinating conversation, he had some really interesting thoughts on it. He worried for the kids who might be left back by too much in that method, for example. Yet he noted that many kids would be able to work at a higher level in at least one subject.) We have also shared that one grandmother skipped two grades and graduated at 16. We pointed out famous people (historic and current) who have been accelerated. All these seem to make him feel less alone.
