Hmmm. Going off of a human baseline for purposes of age.
1st-level: Inexperienced, lazy, or talentless teenagers. And folk in their twenties who had to spend an excessive amount of time studying and meditating/practicing cantrips/praying as opposed to more active training and practice, like apprentice wizards and neophyte clerics. Or just extremely lazy/underachieving twenty-somethings doing simple labor. A farmhand or apprentice blacksmith maybe. A preteen pickpocket who relies entirely on their 'trade' to survive. An inexperienced recruit in the local guards. A young squire.
2nd-level: Journeyman of a simple profession or craft, someone who can probably manage to scrape by a living for themselves. A talented or hardworking mid-late teens teenager who has trained in their trade since childhood. A lazy fellow in their late twenties or early thirties. A scholar in their mid-late twenties. A farmer in their late twenties, or a very active farmer in their early twenties who gets in a lot of brawls or something and learns from the school of hard knocks. A pickpocket in their early or middle teens. A guard with minor experience or some talent. A capable squire.
3rd-level: A journeyman in a significant craft or profession, like a shipbuilder. A master of a simple profession or craft, like a chandler. A talented or hardworking fellow in their late teens, maybe a bit younger if an adventurer or other person facing frequent challenges instead of repetitive tasks. A capable diplomat or monarch, but by no means hardworking or talented. A capable thief or an experienced pickpocket. A guard of average experience somewhere with little danger, or a talented new guard. Someone in their thirties who does a lot of slow activity like reading and writing, a scholar probably. An experienced squire or novice knight.
4th-level: A master in an intermediate craft or profession, or an experienced journeyman. A talented teenage adventurer or warrior with significant experience for their age. A lazy but skilled or talented merchant or leader in their thirties or so. An experienced, respected scholar or artisan. A skilled thief or great pickpocket. A guard captain in a fairly peaceful place, or a lieutenant in a more active guard or military unit. An average young knight. A farmer who's seen more than his fair share of wolves, barfights, and other troubles.
5th-level: A master of a major craft or profession, though probably not so great he or she would be famous. An old, lazy monarch or a talented young monarch in the midst of his or her reign of conquest. A peaceful place's veteran guards. An old farmer or something. The leader of a small military unit or guard unit in a major city. An experienced young knight. A middle-aged but talented or skilled individual. A well-known scholar. A small temple's head priest. A guildmage of middling rank. A monastery's ranking subordinates to the master monk, in a small or new monastery. A small dojo's young sensei. An accomplished big-city thief, but no famous scoundrel. The first mate on a small pirate-ship, or the bosun of a larger ship maybe.
6th-level: An experienced and well-known master in a craft or profession. A venerable but active monarch of only minor accomplishments. An elite guard to such a monarch. A village's champion or veteran militia leader. A well-known knight with talent or great experience. An adventurer in their twenties or early thirties, depending on successfulness and studiousness or activity level. A master thief. A typical guildmaster. A great scholar. An experienced but untalented pirate captain barely worthy of the rank. A large military unit's second-in-command. A great diplomat or capable spy.
7th-level: A famous and highly experienced master in a craft or profession, probably getting old. A war-leader monarch who needs to command on the battlefield to maintain his or her position or expand the territory. An adventurer in their late twenties or early thirties. A guildmage of upper-middle rank. A master sage. A capable assassin or great spy. An accomplished knight. A city's constable or a large military unit's leader.
8th to 10th-level: Highly experienced adventurers and warriors, or talented and highly accomplished old civilians. Royal guards for a great monarch. Upper-ranking guildmages in small or average guilds. Experienced assassins and master spies. Successful and experienced warlords. Veteran pirates or talented middle-aged pirates.
11th to 14th-level: Very talented adventurers, warriors, or leaders with great experience. Elite royal guards, the royal wizard, or an old and very accomplished monarch. Master mages and minor archmages. Major temple leaders. Knight-champions leading an order of knights. Infamous pirates and mercenaries. Master assassins.
15th to 20th-level: Legendary figures and living legends. Veteran adventurers. Veteran warriors of rare talent and great skill. A kingdom's or religion's champion. An old pirate-king or pirate-queen. Challengers of minor demigods or quasi-deities, archfiends, elemental lords, and such.
21st-level onward: So legendary they're probably thought to be mere myths, or at least so legendary that their legends have grown far out of proportion. Only rare individuals make it so far, generally the best of the best adventurers who never quit and never settled down. Occasionally just an ancient and accomplished, but not very ambitious, elf or other long-lived person.
I consider things like 20th-level commoners to be non sequiturs, total nonsense. They're not likely to go above 10th-level, because their lives are too sedate and low-risk, so there's no way they'll learn or accomplish as much in their lives as a live-dangerously adventuring prodigy would. Maybe, just maybe, an immortal elf might become a 20th-level commoner eventually, but D&D elves aren't generally immortal.
