Levels are a really weird. They can be fun, but totally gamey and abstract.
It's totally believable that in a kingdom at peace, the king's champion might have a lot of training without having engaged in deadly conflict. The untested warrior...
Levels in themselves are weird (and "gamey") because of the way they group together, and homogenise, various elements of human capability.
Ability scores are similar in this respect (eg there are at least some human beings who have good reaction times but don't actually move very fast, and/or have poor balance; but in D&D these traits all tend to track one another via DEX). But it's especially notable in respect of levels - all those wise mages also tend to be at least reasonable knife-fighters!
(The gaining of hp and saving throws with level is a more complex matter because those things are also "meta" aspects of the mechanics - Gygax in his DMG (pp 111-12) describes increased hp and saving throws of manifestations of divine intervention.)
But as well as levels, there is also the issue of XP - and D&D XP are much more obviously a gameplay device. From Gygax's DMG (p 85):
Players who balk at equating gold pieces to experience points should be gently but firmly reminded that in a game certain compromises must be made. While it is more "realistic" for clerics to study holy writings, pray, chant, practice self-discipline, etc. to gain experience, it would not make a playable game roll along. Similarly, fighters should be exercising,
riding, smiting pelts, tilting at the lists, and engaging in weapons practice of various sorts to gain real expertise (experience); magic-users should be deciphering old scrolls, searching ancient tomes, experimenting alchemically, and so forth; while thieves should spend their off-hours honing their skills, "casing" various buildings, watching potential victims,
and carefully planning their next "job". All very realistic but conducive to non-game boredom!
Gygax's PHB also tackles this (p 106):
Gaining experience points through the acquisition of gold pieces and by slaying monsters might be questioned by some individuals as nonrepresentative of how an actual character would become more able in his or her class. Admittedly, this is so, if the existence of spell costing clerics, druids, magic-users, and illusionists is (unrealistically) granted; likewise,
dwarven superheroes, paladins, elven thieves, half-orc assassins, and the like might gain real experience from altogether different sorts of activities. This is a game, however, a fantasy game, and suspension of disbelief is required. If one can accept the existence of 12' tall giants, why not the rewarding of experience points for treasure gained? While praying and
religious-oriented acts are more properly the activities for which a cleric would gain experience points, this is not the stuff of exciting swords & sorcery adventure. So too, fighters need physical training and weapons practice, magic-users long hours of study in tomes of arcane lore, and thieves the repetition of their manual skills and discernitory prowess; but none of this is suitable to gaming. It is, therefore, discarded and subsumed as taking place on a character's "off hours".
Ultimately, in AD&D XPs are victory points. And there's no reason to extrapolate this player-side victory condition to NPCs. Gygax's DMG even allows for levelled NPCs incapable of gaining levels (eg mercenary officers - see pp 30-31). In these cases level is a measure of ability, but it is disconnected from the
gameplay activity of earning XP by succeeding at the game.
I don't profess to understand what exactly XP are in 2nd ed AD&D, 3E or 5e - clearly not a measure or marker of any ingame causal process, but not exactly victory points either given the way those games are to be played (at least as their rulebooks present them). In 4e XP aren't victory points either - they're clearly a pacing device, accrued not by
winning but simply by
playing and engaging the situations with which the GM confronts the PCs (and, thereby, the players). Needless to say, in 4e a King's Champion NPC could be of whatever level seemed fitting (by default low-to-mid paragon would seem right) and could have whatever background seemed applicable. XP would have nothing to do with it.
It is possible to have "simulationist" XP - Rolemaster has a version of this - but of course in this case they are earned in quite different ways from AD&D (eg not for treasure gained; and for a whole range of successful endeavours, not just fighting). But the most obvious route for a "simulationist" game to take is to drop level and XP altogether in favour of discrete measures of discrete capabilities - which is what RuneQueset, Traveller and Burning Wheel all do. In any such system, a king's champion NPC would need to be presented in a fashion that accords with the game rule's conceits about how people learn; but there would be no assumption that s/he "levelled up" by raiding dungeons, killing orcs and taking their treasure!