There are 4 Animal Stages:
Baby animals - They will become available after you have unlocked the Breeding Station at level 14. You can get them from breeding Mature animals of the same breed. They do not produce any animal products and need time to grow up into an Adult animal.
Adult animals - They will be introduced to you early in the tutorial of the game.
You can also get Adult animals by growing Baby animals after you have unlocked the Breeding Station at level 14.
Adult animals will start producing specific animal products every time you feed them, and they take a shorter time to produce compared to the animal products produced by Mature animals. Tap the animal and then tap the (i) icon to see how many feedings they need before turning into a Mature animal.
Upon feeding an Adult animal, you will have to wait for the timer to finish in order to get the animal product. You can also use Gems to bypass the timer.
Note: Experience Points (XP) are earned when you acquire either a Baby animal or an animal that is already an Adult from Orderboard Chests, Store, event rewards, leveling up, etc. No XP is acquired when a Baby animal matures into an Adult animal.
Mature animals - Adult animals become Mature after a certain number of feeds, and the time they take to produce animal products returns to the regular pace.
Once you have unlocked the Breeding Station, you can send Mature animals there to breed Baby animals. To learn more, please visit: How do I breed animals?
Mature animals will be replaced by an Adult animal if you either have them in your Barn or upon getting a new one. If no replacement is available, the Mature animal will stay in the Animal Home until you breed them.
Elder animals - Mature animals will become Elders once their Baby becomes an Adult, and upon turning into Elder animals, they will automatically be sold for Elder Animal Points until you reach Level 14. To learn more, please visit: What are Elder Animal Points?
Note: Baby and Adult animals can’t be sold.