BOF Rule 2.4.1 wrote:Competitors are divided into classes according to their age and gender.
BOF Rule 2.4.2 wrote:Competitors’ orienteering ‘ages’ are defined as their age on the 31st December of the year of the event. Competitors therefore change from one age group to the next on the 1st January of the calendar year.
BOF Rule 2.4.3 wrote:Competitors aged 20 or younger belong to each class up to the end of the calendar year in which they reach the given age. They are eligible to compete in older classes up to and including 21.
BOF Rule 2.4.4 wrote:Competitors aged 21 or older belong to each class from the beginning of the calendar year in which they reach the given age. They are eligible to compete in younger classes down to and including 21.
BOF Rule 2.4.5 wrote:Women may compete in Men’s classes but Men may not compete in Women’s classes.
BOF Rule 2.4.6 wrote:A competitor may enter and compete in only one class at any one event.
BOF Rule 2.4.7 wrote:At events where British Orienteering age classes are used they shall be as follows (see image below),unless defined otherwise within the relevant Event Rule.

Juniors have to be under the maximum age, not between the age limits. Seniors have to be over the minimum age, not between the age limits. The 21 courses are open for all. Women can run the men's course.
BOF Rule 2.4.7 would come into play in night events, for example, where young juniors are not allowed to run for their own safety.