aside from the political comments above, i'd like to comment as a parent.
It would seem to me as a parent that this change has come about as a result of teenagers wishing to move up to senior races whilst technically still juniors ( i have infact seen and heard organisers condone this during the year)
i have two daughters 9 and 7 , both girly girls, both slightly built, both city girls..
The eldest has this year competed many times in the u9 races . they have been an appropriate length and difficulty ( mostly grassy slopes rather than rocks shale) and she has grown in confidence and ability racing in this age catagory throughout the year. she has joined an athletics and triathlon club on the back of this and now is looking forward to the cross country season and next summers fell races as an u12.
I strongly believe though that had the rule change being enforced this season she would not be in the same place, having had to compete in most of her races as a u12. The races are significantly longer for a young absolute beginner and the route often much more testing and I strongly believe she would not have have enjoyed the experience.

I suspect from the kids point of view (one rarely considered) they'd have much preferred an age structure based on thier school year, one with which they are familiar and at ease.

to expand even further, policing of ages is vague at best... this summer we have personally witnessed one young person run as U10, U12 and U9 in that order in the same month!