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There are lots of ways to extent the life of a Sim. There's eating Ambrosia, drinking Young Again potions, and others. However, from what you said, I think what you're looking for is in the Options Menu (the button in the lower left corner with three dots on it). Click that, then click on Options, then choose the Game Options tab. On that window, you can specifically choose how long each lifestage will be. Just slide it to the left or right until it gives you the number of days you want. I hope this works for you.