Adult

Adult is a life stage in The Sims and The Sims 2, It is the longest life stage in a Sim's life. In this stage, Sims can get children and start a family of their own. Adults will be featured in The Sims 3, as a life stage between Young Adult and Elder.

The Sims
In The Sims, being an adult is permanent, as they never get 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 the longest life stage and also the most important. It's the only life stage where your Sim can get pregnant (sometimes, even if your Sim is a male, after getting abducted by Aliens) and get a normal job. In the Freetime EP, your Sim can study up on parenting via the book case. The adult stage lasts up to 29 days, but adult Sims can drink life elixir to slow down the aging process.

The Sims 3
In The Sims 3 the Adult stage is no longer the default age. Young Adult is the default age. Adults in The Sims 3 are more "Middle Aged". They are probably 30-59 years old. They have wrinkles. There's not much difference between an adult and Young Adult, they can still get married and have children, but young adults are more fertile.

Adults have 5 traits and takes 21 days (on the normal life span setting) to progress to the next life stage: Elder.