Instance levels (Pre BC)

10s
Ragefire Chasm 13-18 Orgrimmar, Kalimdor
The Deadmines (VC) 15-28 Westfall, Eastern Kingdoms
Wailing Caverns 17-24 The Barrens, Kalimdor
Shadowfang Keep 18-32 Silverpine Forest, Eastern Kingdoms

20s
The Stockade 22-30 Stormwind City, Eastern Kingdoms
Blackfathom Deeps 24-32 Ashenvale, Kalimdor
Gnomeregan 24-40 Dun Morogh, Eastern Kingdoms
Razorfen Kraul 29-38 The Barrens, Kalimdor

30s
Scarlet Monastery: Library 34-38 Tirisfal Glades, Eastern Kingdoms
Scarlet Monastery: Graveyard 34-38 Tirisfal Glades, Eastern Kingdoms
Uldaman 35-52 Badlands, Eastern Kingdoms
Razorfen Downs 37-46 The Barrens, Kalimdor
Scarlet Monastery: Armory 38-45 Tirisfal Glades, Eastern Kingdoms
Scarlet Monastery: Cathedral 38-45 Tirisfal Glades, Eastern Kingdoms

40s
Zul'Farrak 44-54 Tanaris, Kalimdor
The Sunken Temple 44-60 Swamp of Sorrows
Maraudon 46-55 Desolace, Kalimdor

50s
Blackrock Depths 52-60 Blackrock Mountain
Blackrock Spire (Lower) 52-61 Blackrock Mountain
Scholomance 56-61 Western Plaguelands
Stratholme 56-61 Eastern Plaguelands