It's unusual for cats to run away without good reason (i.e. they arn't being treated well). Not to say that would be the only reason a cat may not come home - it could have found a new home, it could have been killed (sad to say, but it does happen)
that is true My cat ran away few weeks ago, he is only 8 months old but after two weeks he came back home, I was really happy! he was probably in someone else's house and couldn't go but than he had a chance came back!