The kids are getting less fit than even their older siblings
The Army has had to increase basic training by about one day per year since the 1960's to ensure that kids get to the same level as they were 50 years ago, hence basic training is now 6 weeks longer - the fitness test has not changed.
We also noticed that a group of Explorer Scouts (aged about 16-18) took exactly half as much time again to do a multi-day Alpine circuit as their identical compatriots had taken 15 years earlier.
When I was in my early 20's I would regularly beat my middle aged Leaders to the top of mountains, we all could, and the Leaders expected it.
Now I'm middle aged I have yet to be beaten to the top of a mountain by a 20 year old, they simply don't have the stamina of the kids of 20-30 years ago.