After looking around a bit, there seems to be a level range on all wild pets in a zone - Here's what I found so far...
Eastern Kingdoms
------------------------
Dun Morogh, Ironforge, Tirisfal Glades, Eversong Woods: Levels 1 - 2
Ghostlands, Silverpine Forest, Redridge Mountains (only 4 - 5), Loch Modan: Levels 3 - 5
Duskwood, Wetlands, Hillsbrad Foothills: Level 6
Arathi Highlands, Northern Stranglethorn: Level 7
Cape of Strangelthorn, Hinterlands: Level 9
Western Plaguelands: Level 10
Eastern Plaguelands: Level 12
Searing Gorge, Badlands: Level 13
Burning Steppes, Swamp of Sorrows: Level 15
Blasted Lands: Level 16
Kalimdar
-----------
TBD
Outlands
-----------
TBD
Northrend
-------------
Borean Tundra: Levels 19-20
Sholazar Basin: Levels 20-21
Icecrown: Level 22
Cataclysm
-------------
Twilight Highlands: Level 23
Pandaria
-------------
TBD
Bugged Zones (I think...)
-----------------
Westfall (only one single Tiny Harvester and it spawns underground), Elwynn Forest (no wild pets), Stormwind (no wild pets)
Zones without wild pets
-----------------------------
Tol Barad, Wintergrasp
I'll try to update this when I find out more zones...