We will soon be applying a hotfix that causes level-25 pets to no longer “steal” experience from your lower-level pets. Previously, if you had a level-3 and level-25 pet grouped together for a battle, the XP earned would be shared between the two at the end. This resulted in some players doing what they could to kill off their level-25 pet so the level-3 pet would get all of the experience. You no longer need to worry about this as the level-3 pet will now receive 100% of the XP that’s earned, so long as it has participated in at least one round of combat.
With max-level master trainer dailies available for Northrend, Cataclysm, Pandaria, and the new line of Spirit Trainers, you have quite a few endgame battles in Pandaria which you can use to level up new pets. Be sure to wear your Safari Hat to get the most out of your battles! The over-arching goal is to provide a variety of content with which you can level up your pets each day.