The numbers on the Left are from the WoW Site. The Order and numbers on the Right are from the Instances_by_level page. The first number is the lowest level that may enter the range is the recommended level range. Since Our group is limited by the lowest player I am only listing the Old World instances. also see the Instance page.

13-20	 Ragefire Chasm (Horde) 8 (13-16)
 	
16-24	 Wailing Caverns 10 (17-21)
 	
17-25	 Shadowfang Keep 10 (18-21)
 	
16-24	 The Deadmines (Alliance)10 (18-22)
 	
20-28	 Blackfathom Deeps 19 (22-24)
 	
21-29	 The Stockade (Alliance) 15 (23-29)
 	
23-31	 Razorfen Kraul 17 (24-27)
 	
28-44	 Gnomeregan 15 (25-28)
 	
33-41   All of The Scarlet Monastery

	 The Scarlet Monastery#The Graveyard 20 (30-32)
  	
	 The Scarlet Monastery#Library 20 (33-35)
  	
33-41	 Razorfen Downs 25 (34-37)
  	
	 The Scarlet Monastery#Armory 20 (35-37)
  	
	 The Scarlet Monastery#Cathedral] 20 (36-40)
  	
36-44	 Uldaman 30 (36-40)

42-50	 Zul'Farrak 35 (43-46)
  	
40-52	 Maraudon 30 (43-48)

45-54   (ST) Temple of Atal'Hakkar 35 (47-50)
	
48-60   Blackrock Depths 40 (48-56)  
	
        Dire Maul:East 45 (55-58)  
  	
53-61   Both Blackrock Spire

        Lower Blackrock Spire 45 (54-60)  

54-61   All: Dire Maul  

        Dire Maul:West 45 (57-60)  
	
        Dire Maul:North 45 (57-60)  
	
56-61   Stratholme 45 (58-60)  
	
56-61   Scholomance 45 (58-60)  
	
        Upper Blackrock Spire 45 (58-60)