Posted this on the official blizz forums too but for my own reference I post here with all my other tips.

Pet Battle Levels with zone levels


Pet Level 1
Azuremyst Isle 1-10 Alliance starting zone Dreanei
Teldrassil 1-10 Alliance starter zone Night Elf
Dun Morogh 1-10 Alliance starter zone dwarf and Gnome
Elwynn Forest 1-10 Alliance Starter zone Human
Durotar 1-10 Horde Starter zone Orc and Troll
Eversong Woods 1-10 Horde starter zone Blood Elf
Tirisfal Glade 1-10 Horde starter zone Undead
Pet Level 1-2
Mulgore 1-10 Horde starter zone Tauren
Darnassus Alliance capital
Exodar Alliance capital
Stormwind Alliance capital
Ironforge Alliance capital
Silvermoon City Horde capital
Orgrimmar Horde capital
Undercity horde capital
Thunderbluff** Horde capital

Pet Level 3
Westfall 10-15
Northern Barrens 10-20
pet level 3-5
Azshara 10-20
Bloodmyst Isle 10-20
Darkshore 10-20
Ghostlands 10-20
Loch Modan 10-20
Silverpine Forest 10-20

pet level 4-5
Ashenvale 10-25
Redridge Mountains 15-20

pet level 5-6
Duskwood 20-25
Stonetalon Mountains 25-30

Pet level 6
Hillsbrad Foothills 20-25
Wetlands 20-25

Pet level 7
Arathi Highlands 25-30
Pet level 7-8
Northern Stranglethorn Vale 25-30
Desolace 30-35

Pet level 9
Southern Barrens 30-35
The Cape of Stranglethorn 30-35

Pet level 10
Western Plaguelands 35-40

Pet level 11
The Hinterlands 30-35
Feralas 35-40

Pet level 12
Dustwallow Marsh 35-40
Eastern Plaguelands 40-45

pet level 13
Thousand Needles 40-45
Badlands 45-48
Tanaris 45-50
Searing Gorge 48-50

Pet level 14
Felwood 45-50
Swamp of Sorrows 52-54

Pet level 15
Moonglade
Burning Steppes 50-52
Un'Goro Crater 50-55

Pet level 16
Blasted Lands 54-58
Silithus 55-60
Ahn'Qiraj: The Fallen Kingdom

Pet level 17
Winterspring 50-55
Deadwind Pass 55-60
Pet level 17-18
Hellfire 58-63

Pet level 18
Zangarmarsh 60-64
Terrokar 62-67
Nagrand 64-67
Pet level 18-19
Blade's edge 65-68

Pet level 20
Netherstorm 67-70
Shadowmoon Valley 67-70
pet level 20-21
Borean tundra 68-72
Howling Fjord 68-72

pet level 21
Grizzly Hills 73-75
Scholazar Basin 76-78

Pet level 22
Dragonblight 71-75
Zul'drak 74-76
Crystalsong forest 77-80
Icecrown 77-80
Storm Peaks 77-80
Pet level 22-23
Mount Hyjal 80-82
Deepholm 82-83

Pet level 23
Uldum 83-84
Twilight Highlands 84-85

Pet level 23-24
The Jade Forest 85-86
Valley of the Four Winds 86-87
Krasarang Wilds 86-90
Kun-Lai Summit 87-88

Pet level 24
Townlong Steppes 88-89
Dread Wastes 89-90
Vale of Eternal Blossoms 90