That's easy.
It's a per-save setting so you'll have to load up a save, go into game settings by clicking on the three dots in the upper-right corner. Click on "Game Options" then "Gameplay" on the left. And you'll see two options for "Auto Age". Set them to No or Unchecked and none of your sims will auto-age.
You can still use the birthday cake to auto-age them up to the next age group yourself.
ThePsycho
____________________________________________________________
*Accept as Solution button - If a post answers your question please let everyone know by hitting this button.
*Me Too button - 'Me Too' helps track how many players are also experiencing that issue. Definitely make use of this button.
*XP button - Say Kudos and help players 'Level Up' by giving them XP for helpful posts.