While I agree with the rest of your post entirely, this quote really irks me. It's nothing but a crude metaphor masquerading as wisdom. People grow old, and as they do, their bodies function less and less well for a series of biological reasons. Empires are not biological organisms, they have no telomeres that get shorter as their constituent parts (people in analogy to cells) reproduce. You can't state that the Roman empire fell because it was old unless you explain why being old makes it weaker. Waving your hand and pretending to be poetic by saying "well, people get weaker as they get old, so this is also true for empires" is not only BS, but demonstrably false.In the end, though, all the countless pages of speculation about why the border collapsed, paticularly in the west, amount to one simple fact: the empire grew old. Adapt though it might, its mechanisms for dealing with with change gradually became set and atrophied, its military 'immune system' needed more and more help from outside, and finally - faced with new generations of vigorous neighbours, who had borrowed from the empire what they needed to give their political system and their cultures strength and coherence - it died of old age.
The Empire Stops Here (Philipp Parker)
People, in the absence of strong external factors tend to have similar lifetimes, about 75 +/- 10 years. Empires however sometimes survive for millennia, and sometimes less than a decade, and sometimes in between. Those are not freak cases, there simply is great variety in how long states last far, without any kind of statistical "peak" in the same way people do. That should make it evidently clear that there is no sense in which an aging empire necessarily becomes weaker.