DIV returns a decimal, both ceil and floor will make that an integer, will make it a string.
Not sure what you are trying to accomplish, but if is someone’s age that you are after, then this will do:
parseInteger(formatDateTime([%CurrentDateTime%],'yyyy')) -parseInteger(formatDateTime($DateOfBirth,'yyyy'))- (if($BirthdayStillToComeThisYear)then 1 else 0)
The function as it's currently written would return a String. If you want the return to be a integer, you need to remove the "toString()" from your current function.