Should it be folk or folks?

Folk is an adjective (e.g. folk music, folk art).

Folk is a collective noun (e.g. the folk are uprising).

Folks is a collection of individual folk. The distinction being that "folk" refers to a mass or a mob -- It is referring to the collection or the mass itself. "Folks" is referring to the plurality of individuals that make up the mass.

In your examples, either old folk (the retired population in general) or old folks (some set of individuals) would be correct.

In the second example, I would go with "Some folks never listen" as it is the individual people who are not listening, not the crowd in general.

Merriam-Webster makes these distinctions for folk or folks (both of which it lists as plural forms of folk):

• plural folk

the masses of people in a homogeneous social group as contrasted with the individual or with a selected class : the great proportion of the members of a people that determines the group character and that tends to preserve its characteristic form of civilization and its customs, arts and crafts, legends, traditions, and superstitions from generation to generation (e.g., the Folk of the Air, the Folk of the Fringe)

• folk or folks plural

a certain kind or class of people -- used with a qualifying adjective or phrase (fine folks, parties for the young folks)

• folks plural

people indefinitely (folks say the house is haunted)