Re: How can I age a sim down on Xbox One?

Try entering CAS by using the button under Likes and Dislikes, as suggested above, or the button under Relationship Preferences. These allow you to make changes that would normally be locked, so they might also unlock the age settings for you. If not, all you've lost is a few minutes for testing. You might also try Change Sim on a mirror or dresser, instead of Plan Outfit.

@Twisted_Beauty00   Hello!  I just saw you tagged me here.  I agree with @Sketch793 .  The likes / dislikes menu worked for me after entering the cheat.  You can also get the cheat menu (commonly referred to as the shift-click menu for PC / Mac).  I learned about this somewhat recently myself.

For PS4 you hold circle and press X on a sim.

I don't have an Xbox but I think it should be hold B and press A?  Perhaps an Xbox user can confirm.

Once the cheat menu opens, select the option to Modify in CAS.