Adult is the fifth life stage, between Teen and Senior, that can be unlocked at any level at any point in the game. When players first start up their game, the first Sims they will move in will be an Adult.
This stage is already unlocked from level 1 onwards. This category allows for such things as having different types of relationships, work, hobbies and providing money for the household. By finishing the Adulthood quest, players can age their teens to adults. If you move in a new sim in a house, it will be an adult.
Adults can do many hobbies. And do many things.
Adults do most of the things in many quests.
Adulthood is a quest for teenagers to age into an adult, but before that quest, you can already have adults in any level.