If a gala is run on an 'age on 31st December' basis (e.g., our Club Champs) it refers to how old the swimmer will be on the 31st December in the year of competition. It does not mean the age of the swimmer on the day of the competition, which may be different. So for example, if a swimmer's birthday was the 20th December and they turned 12 on that date, they would swim as a 12 year old in the Club Champs even though at the time of the events (October/November) they were still 11 years old.