Birthday

Birthdays occur when Sims go to their next stage of life. Whenever a Sim is two days away from the next age transition, a warning will pop up in the top right corner alerting the player that there are only two days to either get the Sim's aspiration score up (if the Sim is in the negative), or that there is nothing to worry about since the Sim's aspiration is already high.

Twenty-four hours after that warning, the player will get another alert stating that the Sim can age. The player can either buy a birthday cake in buy mode or the supermarket, or click on the Sim (as long as they are not babies ) and choose "Grow Up". In order for a baby to grow up, a Sim (teen or older) must take the baby to a cake or wait until they (autonomously) get the interaction to "Help Grow Up" eventually.

Sims grow up well or grow up badly depending on their aspiration score at the time of transitioning, how well they did in school or learned toddler skills. A Sim growing up well will have player chosen traits, while a Sim growing up badly will develop a negative trait.

In The Sims 4 when the sims don't blow out their candles or never have a party on time they will grow up by themselves and they'll be sad and have the "Forgotten Birthday" moodlet

Progression
Sims progress from birth, through infancy and on to old age and, eventually, death. The following chart shows which life states are available in each game. Unless otherwise noted, Sims progress through each life stage in order from left to right.

Player tips

 * Always buy cake for babies because of the time restrictions on when a Sim ages, Sims may be constantly interrupted by the autonomous interaction but never succeed in aging the baby.
 * Wait until a Sim's mood is not in the red zone before aging them.
 * Throwing a birthday party and blowing out candles on the cake will give two good moodlets.