If you are on pc you can shorten lifespan just for pets in options or use cheats. First: shift Ctrl and c. Press them all at the same time. Then type testingcheatsenabled true EXACTLY how I typed it. Then, shift and click the sim or pet you would like to age and click Trigger Age Transition with a little orange circle next to it. This means you can do it without your sims or pet having to do it. Then to turn of cheats: Shift + C + Ctrl all at same time and type testingcheatsenabled false. This should turn OFF cheats. There you have it! If time is paused, you will have to play the time ( any time ) And wait for the party to start!