How do you get a cub to stage up?

During stage one, a cub will not leave your den. You will have to bring small animals such as rabbits, Raccoons, hares, and skunks back to it inside the den. If memory serves right it will need to eat ten or eleven times to progress to stage two. When you kill and animal, it will show you how many times you can eat on it. Raccoons and skunks yield four while rabbits and hares are only three. To take the animal back to the den, you must kill it, then walk up to it, and use the attack button again (this may take a few tries as the mechanic is a little finicky) once you have picked it up, you can run back to your den. Upon entering the den, your character will automatically drop it and the cub will automatically start eating it.

Once you are on stage two, things go by much easier. Your cub can leave the den, but it cannot fight. For the quickest results, fight bosses, like the Moose or Bear, and the xp from defeating them will go towards you and your cub both, as long as it is present during the fight.

Once your cub is at stage three, it can fight and you can play as it, but it is slow and weak, usually only doing three or four damage. Repeat what you do in stage two, and the cub should reach the end of stage three easily.

Once it is ready to grow up, there will be a button that says "Grow Up!" Under the character in the menu. If you never press the button, the cub will never grow up.

WARNING: If you hit the grow up button, you cannot undo it. If you fight against really high levelled enemies or bosses, the cub may fly through all three stages and bypass the grow up button, becoming an adult on its own. (This happened to me personally)