This is a pretty straightforward situation. Death occurs if all the following are true:

• Sim's hunger is -99 or below
• Sim is not an employee
• Lot is not community, secret vacation lot, hotel, hobby lot, or witch lot
• Sim is teen or older

Sims with low hunger who don't fit the other categories will have their hunger reset to -95. Sims who are swimming at the time actually die of drowning rather than hunger.

Rate of hunger decay varies by season (lower in the winter), sim's Active (active sims get hungrier), sim's age and a few other things I don't understand yet. For sake of comparison, typical hunger decay rates are:

• Baby: 150/day
• Toddler: 180/day
• Child: 140/day
• Teen: 210/day
• Non-Pregnant Adult: 209/day
• Pregnant Adult: 350/day
• Elder: 125/day

However when hunger drops below -85, hunger decay is reduced to -10 per day in all cases. This means once a sim reaches desperation, it will still take at least a day and a half for the sim to die of starvation through normal decay.