I lean toward the opinion that limiting the abilities of D&D societies to the equivelant of medieval europe handicaps more than it helps. Think bigger population bases & stronger, more efficient economies.
IMC, these support moderate standing armies supplemented, in times of need, with feudal-type militia levies drawn from citizens required to donate service in return for the protection & benefits they enjoy the rest of the year.
I'm also of the opinion that 1st level NPCs of any type are rarer than their higher level counterparts. 1st level NPCs are barely trained, inexperienced rookies and apprentices. By the time someone has spent a half-dozen years plying their trade, overcoming the challenges of day-to-day life & (sometimes) training in the militia, they've gained experience & levels.
Even in real life, most trades or occupations have jargon to differentiate members by experience. Tradesmen start as Apprentices & may advance to Journeymen, Tradesmen or Masters. Soldiers may be Reserves, Recruits, Regulars, Veterans, Old-timers, Special Forces & so-on.
I find it natural & easy to transfer these terms to D&D level equivelants.
An army IMC consists of a few 1st green recruits just out of basic training. Many more 2nd level regulars & 3rd level veterans with time served & some combat behind them, & 4th - 5th level old-timers. And thats just the grunts! Leaders & officers are drawn from especially talented veterans or nobles that begin their careers as pages, squires & knights.
As many have pointed out, D&D armies are likely to have adventurer-types attached just as modern armies have special forces. I'd also expect magic item arsenals, just as modern armies have hitech weapons.
Just the thing I mean!