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.
On December 31st of that year, they are what’s called “aged out” of the Optimist Class and can no longer race the boat. In addition, competitive sailors are rarely over 50 kgs.