Does the sentence work?

If he was talking about a specific actual person it kinda works. If it's supposed to be an adage or some sort of general principle, then no, it doesn't--it's just cryptic in English.

If he was talking about a specific actual person it kinda works. If it's supposed to be an adage or some sort of general principle, then no, it doesn't--it's just cryptic in English. If it's supposed to mean something like "Anybody who is old but is still living is a thief", it makes no sense in an American (or presumably English too) cultural context.

Are you asking if it is gramatically correct?

Yes, it is.

One, old but not dead, is known as a thief.

It means: A person, old but not yet dead, is known as a thief.

The question is: what is he "stealing"? Could it be time? using up resources like space and medical attention?

Who knows?

It does make sense. Old people have lived a long life, unlike the many people who don't make it into old age. Old people have stolen more life from the cosmos.