Often a cause of confusion! And the situation is often further muddled by event organisers having prize categories which do not relate to the official age categories...
Where we publish age categories in the results they are based on the athlete's date of birth as provided by the organisers and the most common issue is where people believe that they are in a different category because they don't realise that the ages are calculated on a specific date rather than the date of the event.
Oh, and if your category is down as 'YOB' or 'UNK' then it's generally because there was something wrong with the date you entered and we cannot work out your age. Usually it's because you put the current year in there rather than the actual year of birth. You can fix this in MY PROFILE for future events but do also remember to fix it in MY ATHLETES as well so any other events you have already entered get updated.
Triathlon, duathlon and aquathlon events run under British Triathlon/ITU/WTC (Ironman) rules
The official age groups for adults are in five-year bands from 20 (eg 20-24, 25-29, 30-34, etc). All ages are taken at 31st December and NOT the date of the race. Children are arranged in groups under the TriStar label with a minimum age (on race day) of 8 and then (on December 31st) 9-10, 11-12, 13-14, 15-16 (Youth) and 17-19 (Junior).
So, if your age category is showing you as, say, 40-44 and you are only 39 on race day it's because you will be 40 on December 31st and you've 'aged up'. Sorry, but that's the way the triathlon world works!
Age groupings here are governed by UK Athletics and/or the IAAF - and you'll need to check which set of rules the event is being run under!
The UKA rules for athletes under 17 are so complicated that you'll need to take a degree course to work them out but the U20 group are 17, 18 or 19 on 31st August.
Senior athletes are aged at least 20 years of age on 31st August and Masters are at least 35 years of age on the day of the competition. Masters are further subdivided into M35, M45, M55, M65, F35, F45 and F55 categories.
Some organisers use 20-39 as Male Senior and then 40+ as the veteran categories (usually 40+, 50+ and 60+). Women are treated differently so Female Senior is 20-34 and then the 35+ are in the veteran categories (usually 35+, 45+ and 55+).
The veteran categories are often given friendlier names: veteran = 40-49 (M) and 35-44 (F), super veteran = 50-59 (M) and 45-54 (F), vintage = 60-69 (M) and 55-65 (F) with super vintage being 70+ (M) and 65+ (F).