Life stage

Seven Life Stages make up a Sim's life in The Sims 2 (and pregnancy).
 * 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: (30 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).

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.