Cats' lifespans vary a lot due to genetics and living conditions. Cats who live indoors but are allowed outside live about 13 years (as opposed to ferals, who max out at about 4). A cat who stays in the house, has regular vet checkups and dental care, and is fed good food, can live 20 years or more, barring congenital illness or severe injury.

Remember that those are averages, though - it's not the "wear and tear" of going outside making indoor/outdoor cats get old faster; but rather a high probability of sudden or at least rapid death from illness or injury.

Love, Columbine