Welcome to the forum Emilie113.
Aging Options
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When you go into your saved game go to top right and the Options Menu>Game options>Gameplay tab.
There you will see the aging options:
Auto Age (Played Sims) with 3 choices in a drop down menu of:
No / Yes / Only Active Household
Auto Age (Unplayed Sims) with a check box.
If you want all the sims, including the ones you play, to not age up choose 'No' and make sure the check box next to Auto Age (Unplayed Sims) is unchecked.
If you want just your family you are currently playing to age up then click 'Only Active Household'.