I just came across this Reddit post, which lists the following indications (from what I've experienced, they seem correct):
- Haven: 1+ (1-4)
- Hinterlands: 2+ (4-7, 8-12)
- Val Royeaux: 4+ (4-6)
- Storm Coast: 5+ (7-11, 13-15)
- Fallow Mire: 8+ (8-10, 12-15)
- Forbidden Oasis: 8+ (8-12, 5-15, 18-20)
- Skyhold: 10+
- Crestwood: 10+ (10-13)
- Western Approach: 10+ (10-14)
- Exalted Plains: 10+ (10-16)
- Emerald Graves: 12+ (14-20)
- Emprise du Lion: 16+ (16-22)
- Hissing Wastes: 16+ (19-23)
Be sure to head into Exalted Plains around 10-12 - it's a fun zone. I didn't go there until last night, at level 17, which makes everything a bit too easy.
Personally, I stayed in Hinterlands until about 8, at which point the only remaining stuff to do was like level 12-ish. I then went to Storm Coast, Forbidden Oasis and Fallen Mire, where leveling was slow enough to make me around 10-12 before moving on the main quest and finishing off Hinterlands. After that, I started Crestwood and The Western Approach - following the areas in the main quests.
Another note: don't be afraid to move the main quest, I thought I might finish it early but it's paced very well and it unlocks more stuff to do. By the time I got to an important point (after which the game really opens up) I was already 25 hours in and had barely done anything noteworthy.