For Senior events all ages are age on the day of the race. In the case of the Senior British and English Championship races this is stated on pages 100 and 103 respectively.

The details of Rule10 have no relevance to organisers of senior Medium and Long races. All they need to know is that an individual is over 18 on the day of the race and therefore the Senior race entry form does not contain a field for "age on 1st Jan." as the Junior form does.

Moreover the Junior race entry form is unsuitable for anyone over 18 on the day of the race because there is nowhere for the individual to sign and he/she doesn't require a parental signature. As I understand it, parents no longer have any legal responsibility for their children from the age of 18.