Just like so many of the past 'questions' in these threads, this is just purely speculation on my part. Just simply looking at the odds, nine or ten months of the calendar year is usually spent within the school year, thus there is approximately a five in sixth chance of each of their birthdays occurring during their final year in high school. Anything else I say here is just speculation, and is in no way the truth of the matter. Unless someone happens to remember a brief statement in the earlier seasons about either one of their birthdays, that is your complete answer. I personally cannot remember much about the earlier seasons at this point. After all, Disney has been showing nothing but a select few season four episodes for the past three months.
Nothing has been said on Kim's part when it comes to her birthday, though.