Sections below are from ICF Wildwater Canoeing Competition rules
3.4 The first year, a junior competitor can compete is the year that his 15th birthday falls in and the last year he may compete, as a junior is the year that his 18th birthday falls in. The last year he/she can compete in Under 23 category is the year that their 23rd birthday falls in.
SPECIAL RULES FOR JUNIOR AND UNDER 23 WORLD CHAMPIONSHIPS
38.2 AGE LIMITS
38.2.1 In the individual events of K1M, K1W, C1M, C1W, C2W an athlete may only enter and compete in the same event in either Junior or U23. For these events Athletes may enter different events in different age groups.
38.2.2 In the C2M and C2W event an Athlete may enter in both age groups with different partners as long as they comply with Rule 3.4. They may not enter both age groups with the same partner.