When Do sims die in Sims 3?

Answer 1

they die when you dont feed them for a few hours, or if you dont let them go to the toilet every 5 mins

UPDATE:

They can also die of drowning, electrocution, or of old age.

To prevent them from dying of old age, you can either 1- unenable aging in options-game options or 2- spend 70,000 lifetime happiness points and buy a potion that takes them back to a young adult or adult. and let me tell you, gathering 70,000 lifetime happiness points is not easy.

To prevent them from drowning, simply DO NOT SWIM WHILE FATIGUED. You can get fatigued by too much eating, too much excercise, or pregnancy. You will get a moodlet that says "Drowning".

To prevent from electrocution, do not repair something electronic ( such as an oven, dishwasher, computer, laundry cleaner or dryer, etc ) right after swimming, going in the hot tub, sliding down a water slide, taking a bath, washing your hands, or while fatigued. personally i would just call the repairman for the 50 simoleons.