This is from an dystopian anthology I read in the early 1970's.
The world is run-down and brutal, reminiscent of Soylent Green / Make Room Make Room. Everyone has a tattoo on their arm indicating their birth date. You can earn a reward (I think: extending your own life) by killing someone and bringing their arm / tattoo to the authorities.
An old man meets a young boy who is in a theft-and-killing gang (they all ride minibikes) and tries to teach the boy about a better way to live. The old man has a different colored tattoo, much more valuable than the normal. The story ends where the old man intentionally leaves a knife on a table (while cooking) and turns his back to the boy, leaving unresolved whether the boy believed and befriended the old man, or killed him for his tattoo / reward.
