The age limit depends on what distance you are due to swim:
You must be 8 years or over on the day of the event to enter a 250m swim. Every child aged 11 and under must be accompanied by one adult (age 18+)
You must be 12 years or over on the day of the event to enter a ½ mile swim
You must be 16 years or over on the day of the event to enter a 1 mile, 2 mile, 5K
You must be 18 years or over on the day of the event to enter a 10K swim