To ensure parents of children are clear on how the age categorisation works for the juniors, I've given some detail below on how it's worked-out. The principles of categorising ages in this way are set by British Triathlon.

Age Categories and How Aging Works

Your child’s age for the event is determined by how old they are on the 31st December of the year the race takes place.  This means, for example, if your child turns 9 on or before the 31st December 2026, they will automatically be placed in the 9-12 age category, regardless of the fact they maybe 8 on race day.  Similarly, if your child turns 13 on or before the 31st December 2026, they will automatically be placed in the 13-15 age category, regardless of their age on the race day.

We have now split the Junior Triathlon into three age categories. Under 9s; 9 - 12-year-olds and 13-15-year-olds.