Perhaps... seeing as you are 6,000 miles and an ocean away, I'll take the chance you won't show up on my doorstep wanting to arm wrestle...
Just saying mental toughness and resiliency is what really matters, really in almost everything, but war especially, and generally it doesn't take long to know what you are made of in life... by the time you are 35 you know how "tough" you are, - and most aren't nearly as tough as they think they are, by the time you are 50 you really know your limitations, which is probably why they stopped the draft around 45 during WWI, - men can be fit and possess considerable endurance well into their 60's, if you have a real job, a farmer, miner, industrial worker, rancher etc.. I run 3 miles almost every day, but it is being more self-aware that makes you a bad soldier after 50 (government certainly doesn't care about widowing wives), with all that "energy and vigor" (and a lot less reflection) young men make excellent soldiers if you can convince them they are fighting for something worthwhile (which is almost never the case - which is why "government" typically wrap wars in flags and other "patriotic" abstractions, - they know that wars are built upon lies and distortions - and WWII was no exception)