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 now the default age in Create-A-Sim. Adults in The Sims 3 can be considered more like adults in their 'Middle-age'. They are roughly 30-59 years old, probably over 35 years old since the main change in comparison with young adulthood is the fertility decrease. They get a few wrinkles, but they aren't as pronounced as they would are in the Elder stage. 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, they still have the option to get a full-time or part-time job. Other than that they are pretty much the same, game-wise.

Adults have the same 5 traits that they received as a Young Adult and takes 21 days (on the normal life span setting) to progress to the next life stage: Elder.