Great Topic, too much for me to cover completely in one sitting, so I'll throw out a few of my (questionably valuable) cents here and there as I can. About me: (For frame of reference purposes)
Training in martial arts since: Age 8 (with some gaps here and there due to injuries, surgeries, having real jobs, etc.)
Current Profession: Personal Trainer(Certified)/Conditioning Coach. (With the occasional Martial Arts, Fencing, and Rock Climbing lessons still being taught as time, and my aging body, allow.)
Well I make no claims to be a warrior but I have a thought or two (being almost 53 myself in one more week):
1) Flexibility and range of motion: This is the one area I have noticed the greatest diminishment of ability. I simply cannot bend, torque and/or twist as I used to. This has implications in training, as well as a fight, because it becomes much easier for you to get hurt or hurt yourself doing simple things that were incredibly easy just 20 years ago. If you happen to wrench something during a serious fight, all the other attributes you may have could go out the window abd be of no consequence whatsoever (e.g. strength).
Good points: It's extremely important to work to keep what flexibility we have, at the very least, as we get older. Better if we can improve our range of motion and balance that with the apropriate amount of muscular strength and power, as well as continued Cardio-Respiratory conditioning.
2) Body weight: At least from what I see in me and a number of close friends, your body wants to add on pounds that are incredibly difficult (close to impossible?) to take off.
Incredibly difficult or close to impossible compared to what? Man, I certainly can't agree with that statement. I'd never expect to hear that sort of talk from anyone who's done hard martial arts training (no offense meant.) I mean if one has survived even a single Ajarn Chai type MT seminar, then keeping weight off or even losing it should seem easy by comparison
Seriously, I deal with this stuff (weight loss/management, exercise/conditioning, etc) on a professional level all day long, and I can tell you without reserve that getting to and maintaining a stable body composition is quite doable at nearly any age for 90% of the population - meaning those without true medical metabolic issues unrelated to lifestyle choices. How "hard" it is to do so, is largely a function of having the right information and then being consistent with your dietary and training habits. Really though, nearly anyone who puts their mind to it can shed the extra pounds and keep them off without too much "suffering" - I mean how painful is laying off the twinkies when compared with getting whacked in the knee or elbow with a rattan stick?
At least getting hit with the stick offers the benefit of keeping your mental toughness and pain threshold high, I'm still trying to figure out what twinkies have to offer.....
(Relatively unrelated side note:) Hey Crafty, that reminds me: What ever happened to Alfonso Tamez? I had the "pleasure" of training with him for a month at the Aspen Academy of Martial Arts back in the late 70's/ early 80's and will never forget the first time he lined us all up and said "You need to feel the stick to appreciate it." I'm sure you know what came next
I loved training with that guy. Is he still around and training/teaching out there? Just curious.
Back on-topic: As Crafty notes, maintaining muscle mass as we age can be more of a challenge than losing fat.