I've been involved with Venture/Explorer Scouts for the last 25 years (so the 14-18yr age group)
Yes the kids are a lot less fit than they were 10 and 20 and 30 years ago, that is not a feeling its a fact, as we can measure past hikes and the miles done,
They get slightly less year on year, and what could be done 30 years ago as standard is now way beyond the physical capabilities of today youth
We also have 'cotton wool' kids, the "if i push him, will he break" mentality, events that we have organised year on year for this age group, starting for about a decade ago we had parents not sure if their little 6 foot hulking teenager was old enough for that sort of thing yet, to such an extent we have dropped a number of events from the program.
I had a local District Commissioner wondering if he could resurrect a well known hike with a Cup and cash price that was instituted in the 1920's for 14 year old boys. It was stopped in the late 1990's as parents would not let their (older) kids do such things (spend the night out under the stars, build a raft, etc.etc.)
So it's not a case of being an old codger, it is a case of thing ain't wot they used ter be
(Just ask the British Army, basic training, which has not changed since the days of the Boer War has increased by one week per decade, every decade since 1920 in order to permit the 'average' 18 year old to reach the basic level of fitness, the original 6 weeks is now 15 weeks and they expect to add another week in 5 years time)