Adult

Adult is a life stage in The Sims and The Sims 2, It is (usually) the longest and theoretically the most important. Obviously, Adults will be featured in The Sims 3.

The Sims
In The Sims, adults are user-created and never grow older. Their lifespan is indefinite and they would need to be killed in some way in order for them to die.

Adult Sims can take up jobs, fall in love, have and raise children, and prepare food, whereas children can do none of these. Obviously, it is nearly impossible for a family to function with no adults.

Adult Sims can pretty much use all of the objects in the game, as well as perform all the interactions with them, as well with other Sims.

The Sims 2
On the Sims 2, the adult stage is (usually) the longest life stage and also the most important. It is the only life stage where your Sim can get pregnant (sometimes, even if your Sim is a male, after meeting Aliens) and get a normal job.