Then click on the cake and select add Birthday candles, and choose the sim you want to age up and have them blow the candles out.
To have a sim age up before their birthday, you will want to make your sim cook a cake. You can manipulate the aging up times by forcing them to age up with a birthday cake. Sims will naturally progress during through these life stages as you play the game. but You can age up babies by clicking on them and selecting “age up.” Once they are a toddler you can start playing as that particular sim in your household. If you have a baby during game play you cannot choose the baby to be your active sim. Sims 4 has six playable life stages: toddler, child, teen, young adult, adult, and elder.
