My daughter skipped kindergarten two years ago (she's finishing second grade now). Last year, in first grade, she got a lot of questions from the kids about why she hadn't done kindergarten. Her answer was simply that she took a test and was allowed to skip kindergarten. Their reply was generally on the order of 'well, you must be smart' and they moved on.
This year the questions were a bit more pointed, and she does get some teasing about her size and age, especially around her birthday. She mentions it, and at times gets upset about it, but it generally passes quickly.
We've talked to her about answering the questions directly but not making a big deal out of the issue, not drawing attention by talking about how smart she is, not discussing placement in the gifted program, etc. When adults ask us about her school placement (which happens much less now, people don't bother doing the age-math after kindergarten!) we answer matter-of-factly and don't make a fuss about it.
