Just for further info.

UKA Rule 107(6)
(6) Seniors
A Senior is a competitor who is at least 20 years of age on 31st December
in the calendar year of competition.

This applies to Track and Field, but interesting that it is felt the age at which someone is recognised as a senior is built in to the rules and that it isn't 18.
For Road 207(5) and Cross Country 507(5) states aged at least 20 on 31st August prior to the commencement of the Competition Year as defined above.

Check out UKA Rule 207 for Road Running and I think you'll find a cause of the problem.

UKA Rules have set age bands for championships such as a Northern 10 Road Champs as we had at the RV10K or the Northern Road Relays based on age as at 31st August.
But for Open Road Races it is age on the day of the race.

This seems to have been "accepted" as the way individual open fell races are dealt with - that age on the day of the race is relevant. It is the way it is presented in the FRA Calendar and it is the way that ROs present it in their own race literature and deal with it on the day.

So I underline again that it is up to the governing body to :-
1. Make sure their rules are clear.
2. Police the ROs to ensure that they are clear and declaring the age limits "within the rules".
3. If 1 & 2 are done effectively then any breaches by any individuals should be dealt with appropriately.

Without 1 & 2 being in place effectively, I cannot see how you can pursue point 3.