Serious running is not for older people. That could have been a less caustic opening. But it is an unnecessary truism anyway.
Not worth disputing.
But it might be interesting to see at what age people think "old"is, for runners or for anyone.
For competitive sprinters its probably (note the modifier) 35+, and for distance specialists maybe 40+.
And those are debatable.
For the average run-of-the-mill Joe or Jill, runner or not, it's very debatable and the rut pad will be littered with exceptions of widely and wildly different examples.
To me 55 is not old OR a "senior." 65 is for anyone taking reasonably good care, diet and exercise and avoidance of extreme politics, is a bit of a stretch--to assign senior status. I'm older so of course my bias shows.
Generalizing is a necessary evil, so one is stuck with it.
So, I'd say some serious updating needs to be done one classifying runners, say "over 35" as "tramps and granny," irregular folks of 55-65 as "seniors." It's about as obsolete as Daylight Savings time and does no one any service.
Move up to catch up, in labeling oldies. Times are a-changing.