Another age bashing troll post. But I'll bite anyways.
I'm basically the guy in your second paragraph that never took running seriously while young. I'm in my late 40's now and the fact that my PRs are all soft, makes running fun as hell. I've setting new PRs at every distance for over a year. Going from a 5K of mid-20s, to low 20s to around 19 (hoping to go sub-19 soon) has been fun. Likewise for the marathon, from 4:00 to 3:47 to 3:13 (first BQ) last month has been really cool (can I go sub 3:10, sub 3:00?) But I'm not exactly setting world records, so I'm sure lots of people would say it's appears 'pointless' (to them).
But I see what you're saying about having super low PRs from earlier years. It would make it harder to enjoy running, but not pointless.
When you're my age, running is great not only for the challenge. It's great because it makes you look good and feel good. Also, for me, it's a time to zone out and listen to music, which means I enjoy the act of doing it. Those are what I'll still be able to take from it when I'm too old to PR anymore.
As you get older, the fork in the road separates further and further apart into "fit" and "f- it" lanes. The difference between a 50 year old running a 20 min 5K and the average 50 year old non-exerciser, becomes HUGE; much bigger than the difference between fit and non-fit 18 year old. With each passing year the separation gets greater.
That's why I'm inspired as hell, when I see a 65 year old run under 8:00 pace for the local 5K. I'm way more inspired by that, than some 18 year old dingus who beat me by 10 seconds without having to try. He doesn't even know what he has. But the 65 year old at 7:59 pace, is DRASTICALLY different than the average 65 year old who can barely get off the couch without a massive grunting effort, a huge BMI and disease rotting away his sedentary body. Fast forward that to an Ed Whitlock running a 2:54 marathon at 73. Damn near impossible!
I'd bet a years salary Kipchogei and Bekele won't be able to do it. Not with 30+ more years of life in the way.
Just wait. As you get older, those 'unimpressive' Masters performances, get more and more impressive as your youth abandons you and takes you further away from feeling young. Already at my age, late 40s, the people who are fit, look and feel 10 years younger. The ones who have said "f- it, why bother" look and feel 10, sometimes 20 years older. And you can tell.
So, it's your choice. Say, "f- it, it's not worth it" and suffer the consequences. Or, find a way to enjoy staying fit as you age, and reap the rewards.
Father time waits for no one, not even you, Prove Me Wrong (OP). So, if you can't stay young, you might as well feel it. And there's no better way than to get out and do a kick a$s run!