newsticker-automatically-mark-items-as-old

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

t

Documentation:
Decides whether to automatically mark items as old.
If t a new item is considered as new only after its first retrieval. As
soon as it is retrieved a second time, it becomes old. If not t all
items stay new until you mark them as old. This is done in the
*newsticker* buffer.

You can customize this variable.