Use a Birthday Cake
Add Birthday Candles to the cake
Have the Sim blow out the candles
Use a Crumplebottom-style Age-Up Cake interaction if available
Use the “Age Up” interaction on a baby
Wait for the Sim’s natural age progression
Enable cheats
Open the cheat console
Enter testingcheats true
Enter cas.fulleditmode
Shift-click the Sim
Select “Modify in CAS”
Change the Sim’s age in Create a Sim
Use a mod that allows instant aging