Life stage

A life stage is the age of a Sim. Sims do not age continuously, day by day, year by year, but all at once, "jumping" to the next stage.

The Sims
In The Sims there are only three ages: Baby, Child and Adult.

Babies grow into children after 72 hours. Children do not grow up into adults, although the Makin' Magic expansion allows magical children to grow up by using the Age of Instant charm. There are third-party programs which can grow a child into an adult. There are also hacked objects which can allow a child to function as an adult - or vice versa.

The Sims 2
There are seven Life Stages in The Sims 2.
 * Baby: (3 days)
 * Toddler: (4 days)
 * Child: (8 days)
 * Teen: (up to 15 days) Note: Teens can grow up early in order to go to University.
 * Young Adult: (varies; usually 24 days) University only.
 * Adult: (29 days)
 * Elder: (varies)

Note: The durations of life stages stated here are subject to two caveats. The first is that the aging off cheat suspends aging for all Sims. The second is that from Teens onwards, Sims can use the Elixir of Life reward object to prolong life by 3 days (if successful) or shorten it by 3 days (if unsuccessful). Drinking from the Cowplant can also prolong life for up to five days.

When a Sim is within one Sim day of transitioning from one age to another, it's their "Birthday", and they can then either wait for 6pm when they will automatically grow up, or players can trigger the age transition themselves with the "Grow Up" self-interaction, or the birthday cake. Babies and toddlers depend on a teen, adult or elder bringing them to the cake, and furthermore babies don't have the "Grow Up" interaction - they always need the assistance of an older Sims to become a toddler, no matter what.

The Sims 3
Stage length in days on Normal Lifespan option setting.

In The Sims 3, Babies and Toddlers has been given 2 and 3 more days. But the others seem to decrease by 1 and 2. Also, Elders will always live a minimum of 16 days. All Sims will now start from Young Adult if default.

There are seven Life Stages in The Sims 3.
 * Baby - 4 days
 * Toddler - 7 days
 * Child - 7 days
 * Teen - 14 days
 * Young Adult - 21 days
 * Adult - 21 days
 * Elder - minimum of 16 days