Respectfully, I think you are making a "classic" mistake in demographics — one of the most counter-intuitive discipline I ever studied.
One TL;DR is that averages give you very, very, very partial information — without at least a median and the standard deviation, it's pretty much meaningless. Math, but applied to stats / demo yields incredibly wrong pictures if you don't look deeper. Think structure, not KPI, and longitudinal trends — what you see today isn't what's "now", a lot of it is what was set up 20, 50, 60 years ago.
Another TL;DR is that the increase in life expectancy is seldom about "how long do people live" but rather "how many of us die too early" — and this is where the fantastic increase in average comes from. The end-of-life expectancy sure rose too, but much less, and is actually starting to fall back down in the US (some other countries too, it's not just Sam, but it's not a generality either).
Thus the ageing of Senators has little correlation with life expectancy, it's a false correlation (again evidenced by biological facts). From a political science / historical standpoint, current epoch looks a lot like e.g. the second half of the Roman Empire ("decadence", although the word is way too negatively connoted if we mean to judge these times through a modern lens — stagnant in influence is more like it, ageing and eating well is another take).
Judging by intuition from having looked at these figures, I'd say the average political apparatus is 20+ years older today than it used to be around the revolution relatively to the active population, in terms of general pyramidal structure (so, beyond averages). Again, not true in all countries — some are even trending younger in Europe, which is about a couple decades ahead of the USA in terms of demographic structure.
Sorry I can't write a book for each of these TL;DR, but the knowledge on how to analyze demographics is there if you Google it.