It's also fair to say that if you allow someone in a race under age, after they have signed a disclaimer, as an RO what would happen then if that under age child caused an injury to another runner.
I could see the RO in deep water there, having already signed up to the Safety Requirements that dictate the accepted age limits.

However, with my other hat firmly on, there is ample evidence that some of the race limits, not just for fell, but for endurance runners progressing through their teens are ill judged.

A young endurance runner of say 14 would be far less likely to come to any grief in say a 4 mile trail race such as they have in Astley Park Chorley, than in a sprint hurdles race.
I also often see 10K race where they have age limit at 16 or 17, when UKA rules state 15.

I actually think that fell running gets it about right with the age and distances. But the closest comparison is XC - our U13 Girls are running around 2.5km / 1.5 mile cross country races in flat parks and yet can run up to 3 miles on fell terrain a U14s.