As far as fell champs are concerned this doesn't look like a major issue, since the championship season starts in the spring and is contained in one calendar year. I would be against a rule that prevented "age on day" being used in one-off races at the organiser's discretion. I know junior athletes who are desperate to race against the seniors in shorter races: those born in January-March would be fed up if they were told they had to wait a further 9-11 months for no good reason.

I agree it would be more of an issue for XC and would initially feel odd as kids would be running with one set of competitors in league races up to Xmas, and then a 50% different set of competitors after Xmas into the county champs, regional and nationals.

Having said that, on a week by week basis they already race with different competitors when they go from racing for their clubs and counties to their schools and county schools. And that's without thinking about eg Kendal Winter League fell racing where there are different age groups again.

The biggest headache might be trying to explain it to parents and getting entry forms filled in accurately.