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

For the love of battle pets

Was trying to find what type of pet to use against beasts since beasts and critters are so very plentiful in the wild battles I have played so far so I made a quick (not really) guide for myself. and I know that there are plenty of helpful charts and more charts but I find the following easier for me to understand.

Locate what you are fighting and use the previous battle pet type for extra damage against your opponent.
Beast vs Critter vs Undead vs Humanoid VS dragonkin vs Magic vs Flying vs Aquatic vs Elemental vs Mechanical vs Beast

Locate what you are fighting and avoid using the next type of battle pet move to avoid low damage moves against your opponent.
Beast vs Flying vs Dragonkin vs Undead vs Aquatic vs Magic vs Mechanical vs Elemental vs Critter vs Humanoid vs Beast