Everything except the trollcave west of the river is appropriate for levels 10 and under. The farmlands are good for levels 10-13.
You should be at least level 10 if you are thorough in clearing out the area around Aleroth, because the enemies after that are all at level 10 or higher.
Everything above ground except the Cursed Abbey on the first map north of the Dwarven village are good for levels 11-17. You WILL run into a few level 20+ corpses and zombies, the latter of which have over 1000 HP, but it's hard to seperate the level 13 enemies from the undead ones, so it's better to just kill them instead of running away and coming back.
The orc battleground and dwarven forest are good for levels 13-19, but the Orc camp is for levels 30-40 so don't get in too deep.
The dark forest and Verdistis sewers are good for levels 30-35, although some groups of mountain trolls are level 45.
Basically, once you're in the farmlands, if you're at level 11 or so, clear out the enemies and start questing.
