This subject is the one that causes everyone the most trouble. The reason for this is that in the southern hemisphere, our season straddles the new year. In the northern hemisphere a season might run from April to October in the same year, hence the calculation for age category is easier to make.
There are MTBA and UCI rules which we stick to at all times.
The rule is:
Our state (and national) series ends in May 2010 - hence the need to be UNDER the age that you enter before the end of the year in which the series finishes.