Another line of reasoning is that the WT itself often refers us back to older publications:
*** w2011 2/15 p. 31 Do You Hate Lawlessness? ***
For a detailed discussion of this subject, see the September 15, 1981, issue of The Watchtower, pages 26-31
*** w07 6/1 p. 14 From Scroll to Codex—How the Bible Became a Book ***
See the article “The Early Christian Codex,” in the August 15, 1962, issue of The Watchtower, pages 501-5.
*** w07 9/1 p. 10 With Jehovah’s Help, We Survived Totalitarian Regimes ***
See the life story of Elsa Abt in The Watchtower, April 15, 1980, pages 12-15.
*** w03 10/1 p. 19 Endurance Under Trials Brings Praise to Jehovah ***
see the 1974 Yearbook of Jehovah’s Witnesses, pages 208-9
You might also ask "at what point should I no longer believe the publications? - At what age do they become unmentionable?"
Depending on how they answer you can add "So when will today's WT instructions become old light that we are to dismiss?"
Splash