Life expectancy for cat?

He is a 9 year old, indoor/outdoor, alley cat. He is indoor/outdoor, but he doesn't cross the street, and stays between the house next door and us. He gets in an occasional cat fight, but wouldn't drink, eat, or go near someone or something that would hurt him. He is really important to me, and I just want to know the average age they live for. I've heard that some cats can live for twenty years, with the right care! Is this true?