I favor a basic advancement rate of 250 XP per year for common NPCs from maximum starting age of their class. This means that most middle-aged adults will be level 3-4. PCs quickly outpace the advancement of older people in the community, and are more powerful because of their higher stats and items. In demi-human comunities, the average level will be higher.
Superior NPCs might get 500 XP per year, starting from the average starting age for their class. Superior NPCs have access to better training and are more active. These NPCs would be rarer, the average adult being 4-5th level. An example of a superior NPC might be a Cormyrean War Wizard.
Elite NPCs, more on the level with PCs, would earn 1000 XP per year, starting with the minimum starting age for their class. They would be rare, and represent characters who either have access to the best training because of their wealth or who have been as active as a PC.
Of course there would still be uniquely high-leveled NPCs as well as incompetent NPCs with lower-than-average levels.
This system I'm suggesting doesn't jive with standard D&D, which assumes every NPC is first level until DM fiat gives them XP. Since experience is earned over time, I think having a standard time-based progression makes sense. I know most DMs just assign NPC levels arbritrarily. Any standard progression scale is helpful for world-building, since it allows you to determine the level of magic that is commonly available. In a 250XP/year advancement, 1st and 2nd level magic, scrolls, potions, and some wondrous items are pretty common in human communities, and 3rd level magic and magic arms and armor are somewhat less available because the average human spellcaster wouldn't reach 5th level until in their 60s.