In my settings there are ambient sources of magic either in the air or related to someone's life force, as someone trains or learns techniques for cultivating this, the limits of our world's physical capability can be broken:
In a way, this is kind of default-- if primal magic is magic, then arguably every living thing is magic and can hone its own life force to be stronger. That's why we have high level foes, who can be natural creatures despite that necessarily putting them on the level of explicitly magical beings.
You could interpret it as ki or whatever too, if we accept the existence of the monk, then rationally there's no real reason why any other martial artist couldn't learn those techniques to reach a similar level of martial ability-- a fighter who learns who to manipulate their ki is literally a better fighter. It dovetails well with the concept of Barbarian Rage as well, there's something inside of you that it draws on and stokes to make you stronger.
Also to be clear, the 14 year old girl from Sacramento can take Orcus provided she trains really hard, somehow without aging. But then again, Wizards often learn 8 of their 9 spell levels in like a year in these games if the party isn't deliberately pacing with long stretches of downtime or time skips to pad it out, whereas its meant to be a lifetime of study and dedication in the lore of the game-- and you can't even compensate for that by putting that behind your character in their history because you still start out without that ability.
Incidentally, speaking of 14ish year old girls from California, Amphibia is very DND, and Sasha Waybright has no patience for not getting to be cool: