Each zone has multiple level ranges for mobs and it can be kind of wonky sometimes but heres a general outline:
Level 1+ Haven (1-4)
Level 2+ The Hinterlands (4-7,8-12)
Level 2+ Val Royeaux (4-6) (Main questline)
Level 5+ Storm Coast (?-11,13-15)
Level 6+ Fallow Mire (6-11,12-15)
Level 8+ Forbidden Oasis (8-12,12-15,18-20)
Level 4+ Mage Questline or Templar Questline
Level 10+ Skyhold (Main questline)
Level 16*+ Hissing Wastes (?-11,13?-23)
Level 10+ Crestwood (10-12,13-?) (Main questline)
Level 10+ Western Approach (10-12,13-14) (Main questline)
Level 10+ Adamant Keep (?) (Main questline, one time visit)
Level 10+ The Raw Fade (?) (Main questline, one time visit)
Level 10+ Castle Caer Oswin (?-12) (Sub quest from Cassandra, one time visit)
Level 10+ Exalted Plains (?-11,13-16)
Level 10+ Lost Temple of Dirthamen (?-11,15)
Level 12+ Emerald Graves (?-11,12-15,16-20) (The Dawn Rises sub quest)
Level 12+ The Winter Palace (12-15) (Main questline, one time visit, recruit Florianne agent)
Level 16+ Emprise du Lion (16-20,18-23?) (The Dawn Rises sub quest)
Level 8+ Cradle of Seluvin (?-11,12-20,20-23)
Level 16+ Shrine of Dumat (16-19) (The Dawn Rises sub quest, one time visit)
Level 16+ The Arbor Wilds (16-19,20) (Main questline, one time visit)
Level 16+ Endgame (16-19)
In general, you want to be at least as high as the min level for an area but ive honestly been able to clear level 16 areas even as level 10 even on NM so there is some wiggle room.