Your Opti age is the age you are on the 31st December of that year. For example: if you turn 12 on the 12th of December you are considered Opti Age 12 for the year.
A skipper can only race Optimists until December 31st in the year they turn 15. After this time they have “aged out” of the Optimist Class and will need to move on to other youth classes such as the P class or Starling. In addition, competitive Optimist sailors are rarely over 50 kgs.