Get your Sim or their parent (if they are too young) to go to the phone and throw a party at home. Make sure it is a birthday party and not just a house party. (This step is optional. You can also skip directly to the second step);
, Go into "Buy Mode". Go into "Entertainment" and then "Party". Purchase a birthday cake.
 When the guests arrive, have the Sim who is going to age up blow out the candles. It will take a while because there will be loads of party blowers and stuff but after all the celebration, the Sim will have loads of sparkly stuff swirling round them and then they will age up.
Start a Party.
Buy the Cake.
Blow out the candles!
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