You are supposed to train it to get it to adult. Take a look at the experience it has, and keep training it until it becomes teen and so on. The experience for it to become a teen is 100, I believe.
After you play a mini game with your pet, you have the option to feed your pet a snack. The actual games don't give much pet xp but the snacks do. It's a good idea to invest in high ranked pet snacks because they give you more pet xp. You can buy them from the bazaar, they're dropped, and some are rewards for quests. Finally, you may just be getting really low scores in the minigames (and not feeding your pet) which results also in slow leveling. Sarai Thunderstrider Master
After you play a mini game with your pet, you have the option to feed your pet a snack. The actual games don't give much pet xp but the snacks do. It's a good idea to invest in high ranked pet snacks because they give you more pet xp. You can buy them from the bazaar, they're dropped, and some are rewards for quests. Finally, you may just be getting really low scores in the minigames (and not feeding your pet) which results also in slow leveling. Sarai Thunderstrider Master
The amount of experience that your pet earns from minigames is dependent on how well you are able to complete the minigame. If you complete the minigame perfectly, you can get a lot of xp, but, if you complete the minigame poorly, you may only receive a little xp (if any). So, play the minigames that you're really good at playing.
Also, you can feed your pet snacks to gain extra xp. The amount of xp received is dependent on the level of the snack (found in the upper left-hand corner, or, for really high level snacks, the sum of its boosts) and whether or not your pet "likes" a snack or "loves" a snack. A pet "likes" a snack if it is of his/her school or if it is of the class that they prefer (candy, desserts, meals, munchies, fruits, or vegetables). A pet "loves" a snack if it is of his/her school and if it is of the class that they prefer.
Lastly, occasionally, feeding your pet high-level items can give some xp.