"The operative date for determining membership of age groups for all athletes under age of 17 shall be... the 31st August at the end of the competition year"

"The competition year ... shall extend from 1st October to 30th September for the following year"

Note: For Cross-Country the dates are the same but refer to the age on 31st August prior to the commencement of that competition year (Rule 507)

Under 11
Those who are aged 8, 9 or 10 on the 31st August that competition year (Mostly those still in Primary school)
(This is based on the UKA rules and Club policy - UKA doesn't specify this age group)

Under 13
Those who are aged 11 or 12 on the 31st August that competition year

Under 17
Those who are aged 15 or 16 on the 31st August that competition year

Under 20 (Sometimes referred to as 'Junior men & women' or 'Juniors')
Those who are aged 17 or over on the 31st August (but under 20 on the 31st December), in that competition year