What ive noticed is that when you give them certian clothing/equippables, there seems to be a certian "level". As in higher quality gear spawns them more towards the higher levels.
Ive tested this very briefly and found that giving your NPCs the "low quality" option gear/clothing will most likely spawn them around the Level 5 to Level 9 mark.
It might be deeper and perhaps everything has certian values that effect at what level your NPCs spawn, but from what i can tell its linked to what your give them.