The 2 juniors are of totally different builds.
The older one is quite strongly built, but #2 is skin and bone.
When he was little, after bath-time, he would scamper about nekkid on all fours, and the likeness to Gollum was uncanny.
He's still the same build ( but we don't see the Gollum-scamperings ).
He struggles on endurance events, because he has no reserves.
Our walk up Half Dome in Yosemite was a real struggle.
Also, he feels the cold easily, which can become an issue when we have -16 days when skiing.
So for him, it was a real achievement, and he's feeling well made up that he's ridden 100k+.