Even if the age cut off went by calender year, you'd still have up to a year in age differnce between the kids anyway? Theres really nothing much you can do to remove the age gaps. Also, if you move states, you end up with big age differences too sometimes, depending on where you are going to/from.
A year gap is always going to happen, but it is much better than 16 or 18 months, which is what is happening at the moment depending on the state. At least at year 12 all students will turn 18 at some stage during the year, as opposed to some turning 18 and another bunch turning 17.