newsticker-automatically-mark-visited-items-as-old

newsticker-automatically-mark-visited-items-as-old is a variable defined in `newst-backend.el'.
Its value is

t

Documentation:
Decides whether to automatically mark visited items as old.
If t an item is marked as old as soon as the associated link is
visited, i.e. after pressing RET or mouse2 on the item's
headline.

You can customize this variable.