The below patch to Gnus allows for scoring based on article age.

For example:

(("date"
  (7 10 nil <)
  (7 -10 nil >)
  (14 -10 nil >)))

results in articles less than a week old to get a 10 point increase,
older than a week a 10 point decrease, and older than two weeks a
cumulative 20 point decrease.

I am also including a documentation change. I have not made any changes
to interactive scoring.