Thanks for sharing, as it’s always fascinating to see one after another over the same fence.
I honestly hadn’t realized this was a competition, vs a training exercise, as some of the YR rode in so tentatively, I think they were praying the horse would stop. Still, better to be slow and safe vs turning turtle in the water!
NOT impressed with the post-refusal whackings - and having to bundle your reins and straighten yourself in the saddle first, in oder to do so - tsk, tsk. Do your schooling at home.
What I liked seeing was the ones who knew their horses well, and rode sensitively.
The dog - minorly annoying, but I doubt the riders and horses even heard it, and they do have to get used to extraneous hubbub on course. I find the hooting and hollering more irksome, but I’m an old grump! 
On the positive side, every horse looked to be of very good quality, and the turnouts were, on the whole, professional. Looks like lots of money being spent on fancy gear (not a criticism - lucky them!).
I wonder how many of them go out regularly with a fast live hunt in the first field on those horses - might do some of them a ton of good, if they want to move onwards and upwards in eventing, as it would help them to think fast while moving at speed, and react accordingly, as well as developing stronger seats.
As for the age thing - it’s not unusual for 14-15 yo to have fairly floppity seats - not a lot of core strength as they’re still growing, but their youth and agility often makes up for it. The world champion show jumper is 18yo at the moment, iirc. And Irish - I believe I read it here!
Lovely rider, # 120 (at 6:52 onwards).
And thanks also to whoever posted the European Young Rider stuff - I predict Cathal Daniels is destined for greatness (OK, he’s already there, it seems, lol) - 1:30 inwards in the first video:
https://www.youtube.com/watch?v=Vf37wIEFVTg&list=PLygX-ItLQtDNYsv_w4LurFRThG2v20o1O