I see this sentiment and rationalization a lot but I don't understand it.

Age aside, presently, are you saying you cannot meet a threshold you would label competitive? Competitive games are almost always played on a spectrum? I would argue your placement in the spectrum should curate the ground for competition if the player base is large enough (and ladder system coherent).

Now with my framing understood, how does age fit in? I can buy that as you age you have less time to put into a game and potentially weaker reflexes (I'm not going to pretend to know the science here), but this should simply inform your placement on the ladder?

I don't think it has anything to do with "people who play 6 hours a day and are in their peak twitch-reflex years" unless you mean your enjoyment is derived from overcoming this archetype.