We're getting this too. ds, who has always loved nursery, said today that he didn't want to go because he hasn't got any friends any more, and they're all babies in pre-school now.
Purely by chance there's a full 6 months age gap between ds (who only missed going to school this year by a couple of days) and the next oldest child in his pre-school, and they've also had a big rush of August 02 children moving up. ds has never actually been in the same group as any of these before, as he was moved up to pre-school early, because he was obviously ready for it well before he was three, so he's always been a part of the group who have now moved up to school.
I can sort of see ds's point, because the new just 3 year olds do seem an awful lot younger than he is. While he's writing, knows his letters, is desperate to know how to spell words, loves basic arithmetic, etc, all the newcomers must seem to him to still be at the scribbling stage - hence his assessment of them as "babies".
What do I do now - ask his nursery teachers to keep an eye on him because he's feeling like this and to make sure he is still given activities which will keep him challenged and interested? I think they will probably not be surprised, as just before his friends left they told us that he was "more ready for school than most of those who are going this year".
Or do I look at moving him somewhere else so that it seems to him he's going to "school" (which I don't really want to do as I love the setting of his current nursery) and then risk having the upheaval of losing new friends again next year?
Or call the parents of some of his old nursery friends and arrange some weekend playdates so that he realises they are still his friends? But would that make things worse to see them and then have them disappear again leaving him at nursery without them?