Like Sims, pets age in stages.
A pet starts as a kitten, puppy or foal[TS3:Pets] and ages to an adult. In The Sims 2: Pets, The Sims 3: Pets and The Sims 4: Cats & Dogs, an adult pet will eventually age to an elder, and will eventually die. However, in The Sims: Unleashed and The Sims 2 Pets for console, adult pets live indefinitely.
In The Sims: Unleashed, kittens and puppies are objects, and appear as maternity tents topped by a cat face or dog face. The mother will watch them, but it is not possible to interact with them.
In The Sims 2: Pets, kittens and puppies are independent family members with their own needs, though they do not have the Scratch or Chew motives. They are mobile, but cannot climb stairs, and must be carried up and down them. They cannot be taken to community lots. They cannot have jobs or learn commands, but they can learn behaviors. While they cannot get on beds or furniture, they can be praised for not getting on them, and therefore trained to stay off them. This stage lasts three days.
The adult stage lasts 25 days for cats and small dogs, and 22 days for large dogs. The length of the elder stage is 5 to 10 days, and is random. The adult or elder stage can be extended by having the pet eat the Kibble of Life. Unlike Sims, who age at 6:00 pm (18:00), pets age at either 10:00 am or 11:00 am. Elder pets can retire from their career. However, they cannot get jobs.
In The Sims 3: Pets, cats are kittens for seven Sim days, adults for twenty-eight Sim days, and elders for seventeen Sim days, then they die. Dogs are puppies for seven Sim days, adults for twenty-five Sim days, and elders for fourteen days, then they will die. Horses are foals for five Sim days, adults for thirty-five Sim days, and elders for seventeen days, then they will die. All living pets are fully controllable at all life stages, and all living pets can learn new skills at all life stages.
Only adult pets can get pregnant, elder males can make an adult animal pregnant.
In The Sims: Unleashed, pets, like Sims, do not age and have indefinite life spans. They can die, and have their own gravestones, but death is not inevitable for them.
In The Sims 2: Pets, pets will eventually die of old age, that being the only way they can die, as a police officer will take the pet away before starvation. When a pet dies, the Grim Reaper will come to take the animal away. Other dogs on the lot will surround the Grim Reaper in a circle and howl. Cats arch their backs and hiss.
Pet deaths have their own unique cinematics. A dog's death will result in the Grim Reaper throwing a stick and the dog fetching, while a cat is thrown a ball of yarn. When a horse dies, the Grim Reaper will ride it through a portal to the Netherworld.[TS3:P] Pets also have their very own gravestones, and a special memory is also added when a pet dies.
In The Sims 3: Pets, all animals will eventually die of old age as in the previous game. A dog will die after forty-six Sim days, a cat will die after fifty-two Sim days, and a horse will die after fifty-seven days.[N] When a cat dies, the Grim Reaper will dangle a cat toy from his hand and encourage the cat to jump for it until it ascends to the afterlife; when a dog dies, he will direct it into a portal to the next life by throwing a stick for it to fetch, or at times cuddle the dog while they lick his face, and he tosses them into a portal above them; and when a horse dies, he will spawn a large portal nearby, mount the horse and ride it off to the underworld. All animals have unique gravestones.
In The Sims 3: Pets, stray cats, lost dogs, and wild horses will also die eventually and get a gravestone. As they die, the other stray animals will surround them, along with everyone else on the lot.