They joy of different age at date rules!  Some leagues operate on an age at day for swimmers (easy!), some age at end of 30th September (no idea why…) and some age at the end of the year (makes some degree of sense!).  It could be you compete as a 9 ear old one day, then as a 10 year old, depending on the league.  You may also have to ‘swim up’ an age if no other swimmers are available, or you’re simply that good.  Ask Eleanor, Neve, Grace and Amelia who have all competed as open age swimmers before they turned 13!!