AGE CATEGORIES FOR T&F RECORDS
The operative date for determining membership of age groups for all
athletes under the age of 17 shall be for Track and Field  the
31st August at the end of the Competition Year
Under 20 Years Junior Men and Women
(i) Track and Field events for Juniors shall be confined to competitors
who are 17 or over on 31st August within the Competition Year, as
defined in (1) above, but Under 20 on 31st December in the calendar
year of competition.
(6) Seniors
A Senior is a competitor who is at least 20 years of age on 31st December in the calendar year of competition.
(7) Masters
Events for Masters shall be confined to athletes who are at least 35 years of age
on the day of competition.
Taken from UK Atheletics Rules for Competition 2012
AGE CATEGORIS FOR ROAD RUNNING
Senior Men & Women
For Road Running competitions a Senior is a competitor who is
aged at least 20 years on 31st August prior to the commencement of
the Competition Year as defined above. In Road Relay competitions
Junior Men and Women, as appropriate, may compete in Senior
events.
(vi) Masters Men and Women
Road Running events for Masters shall be confined to competitors
who are at least 35 years of age on the date of the competition