Here is a list of zones in Age of Conan, the zones recommended level range and the zone path to get to each of the zones.And also the leveling guide including ...

Pre L20

Tortage L1-20
White Sands Isle L1-20
Underhalls L1-20
Acharonian Ruins L1-20

Hubs

Old Tarantia
Conarch Village
Khemi

L20+

Wild Lands of Zelata L20-38 (From Old Tarantia -> Wild Lands of Zelata)

Khopshef Province L20-36 (From Old Tarantia -> Khemi -> Khopshef Province)

The Maze* L25+ (From Old Tarantia -> Wild Lands of Zelata -> The Maze)

Border Ranges* L29+ (From Old Tarantia -> Wild Lands of Zelata -> Border Ranges)

Conall's Valley L20-32 (From Old Tarantia -> Conarch Village -> Conall's Valley)

Spider Caves* L25+ (From Old Tarantia -> Conarch Village -> Conall's Valley -> Spider Caves)

L30+

Overflow Tunnels (Tarantia Sewers)* L30+ (From Old Tarantia -> Tarantia Sewers)

Blessed Cave* L30+ (From Old Tarantia -> Khemi -> Khopshef Province -> Blessed Cave)

Black Castle* L30-38 (From Old Tarantia -> Khemi -> Khopshef Province -> Black Castle)

Sanctum of Burning Souls** L35-42 (From Old Tarantia -> Wild Lands of Zelata -> Sanctum of Burning Souls)

L40+

The Catacombs L40-66 (Quests around 58+) (From Old Tarantia -> Tarantia Noble District -> The Catacombs)

Cistern (From Old Tarantia -> Tarantia Noble District -> The Catacombs)

The Main System (Fresh Water System) (From Old Tarantia -> Tarantia Noble District -> The Catacombs)

Note on Catacombs, Cistern and Main System - Find quests in the Noble Distict. These will help you FIND the instances.

Armsman Tavern / Arena L40+ (From Old Tarantia -> Tarantia Noble District -> Armsman Tavern / Arena)
2 NICE Sets of quests here. One set for solo, one for group. Great Rewards.

Field of the Dead L40-50 (From Old Tarantia -> Conarch Village -> Conall's Valley -> Field of the Dead)

Frost Swamp L40+ (From Old Tarantia -> Conarch Village -> Conall's Valley -> Field of the Dead -> Lacheish Plains -> Frost Swamp)
Quest in Brannoc Village

Imirian Ravine L40+ (From Old Tarantia -> Wild Lands of Zelata -> Poitain -> Imirian Ravine)