Jim
https://fellrunner.org.uk/documents/...2002-11-18.pdf
Page 1 - see 1(d) which refers you to Page 3 Table 2
That clearly states that the 10km limit is for 16 & 17 year olds.
18 year olds can run in any race and have done including the Three Peaks, Fairfield, Anniversary Waltz... to name a few I know of and they can also run the pairs and nav legs in the National Fell relays.
I think someone has confused the issue due to the FRA Junior top age now being U19. These FRA Junior races are capped at 10km even for the 18 year olds purely because the category includes 17 & 18 year olds.