Deflating D&D Experience Levels
My first edition of Dungeons & Dragons was the 1981 Moldvay Basic but I quickly fell victim to Gygax’s marketing of Advanced D&D. It was years before my brother and I found anyone else who had the game, so all I had was the TSR products as-written. The 1983 World of Greyhawk boxed set was the one and only campaign world, then. The encounter tables in the Glossography became unforgettable as they set the bar for NPC experience levels. The entry for “Men, Patrol, Knights” is the best example of this:

Ignore the high-level officers. The average knight is at least a 4th-level Fighter. His squire is 2nd- or 3rd-level. Even the serjeants are 1st-level Fighters, which is meaningful as AD&D has the concept of “zero-level” for commoners a little better at combat (e.g. militiamen) than the rest. Other entries in those tables have similar experience level distributions.
Regardless of rules edition, these are high levels for average warriors. Knights are elite professionals, true, but all of them in (what 5th-edition D&D calls) Tier 2? D&D presumes that all character classes will be present in equivalent distributions, so Tier 2 spell-casters will also be common. Porting these levels as-is into 5e would not change my point at all. One of my many quibbles with 5e is its continued power inflation from 3e. Even more than with character classes, I disagree with Knights being 8d8+16 HD opponents and peasants having 16 Hit Points.
This has a huge impact upon the rest of a campaign world. How many of us have carried these assumptions into our own custom worlds? It has taken me years (decades!) to truly see that knights, for one, can still be feared killing machines at lower experience levels. That even comes with a bonus: they also become mortal, which every non-hero should be. If you are keeping the spell-caster equivalency, this also lowers the magical power level of your world—a very good thing, in my opinion.
In 5e terms, I see a squire as a 1st-level Fighter. An average knight would only be a 2nd-level Fighter. Higher levels are still available for experienced knights, of course. Against peasants and militiamen, a 2nd-level Fighter is a sufficient killer but is also at risk of being slain by a few enemies working together.
For further discussion on this topic, I highly recommend JB’s (B/X BLACKRAZOR) blogposts Hit Me Baby One More Time, 1st Level Magic-Users, and One Man Army.