Buffy and Dawn are roughly 5 1/2 years apart. Unless you want Buffy going to school on the weekend and MLK, Jr. Day, her birthday has to be January 21, 1981.
Dawn's pretend birth year was 1986 (summer birth date most likely due to when her stated age changes--she appeared in August 2000). Connor's pretend birth year was similarly 1985, but was born the night of November 29, 2001 (full moon the next night).
Dawn should be 24-25, depending on part of the year 2011. She was 14 in all of season 5, as she was 15 in season 6 and 16 in season 7. She attended her last year of high school in Europe.