Best I can figure, the game starts out with all the different regions at a particular range of levels shown on the map. As you level up, the enemies in the area level up with you up to the maximum of either…
- The region cap, or
- Your level - 4.
Pretty much everyone I run across (except occasionally for main quests) is 4 levels below me now.
Note: I’m playing on Easy, so YMMV.