Craig Withers

@Eoin Joyce quoted one of the first real hints of the overlapping generation doctrine from the Feb 15, 2008 WT (above).
Above, I also quoted a WT QFR that mentioned overlapping generations from the Sept. 1, 1952 WT, and it showed why overlapping generations do not effect the length of the generation Jesus spoke about.
After the 2008 article, there was also a more recent mention of the some of the same points repeated in the April 15, 2010 WT, and this 2010 article was the first to explicitly create a generation that is defined by the overlap of two specific groups: One group saw the start of an event, and another group could not have seen the start of that event, but would see the start of a different event, not more than two lifespans from the first event:
*** w10 4/15 pp. 10-11 pars. 13-14 Holy Spirit’s Role in the Outworking of Jehovah’s Purpose ***
13 Third, holy spirit is at work in bringing Bible truths to light. (Prov. 4:18) This magazine has long been used by “the faithful and discreet slave” as the primary channel for dispensing increased light. (Matt. 24:45) For example, consider our understanding of those who make up “this generation” mentioned by Jesus. (Read Matthew 24:32-34.) To what generation did Jesus refer? The article “Christ’s Presence—What Does It Mean to You?” explained that Jesus was referring, not to the wicked, but to his disciples, who were soon to be anointed with holy spirit. Jesus’ anointed followers, both in the first century and in our day, would be the ones who would not only see the sign but also discern its meaning—that Jesus “is near at the doors.”
14 What does this explanation mean to us? Although we cannot measure the exact length of “this generation,” we do well to keep in mind several things about the word “generation”: It usually refers to people of varying ages whose lives overlap during a particular time period; it is not excessively long; and it has an end. (Ex. 1:6) How, then, are we to understand Jesus’ words about “this generation”? He evidently meant that the lives of the anointed who were on hand when the sign began to become evident in 1914 would overlap with the lives of other anointed ones who would see the start of the great tribulation. That generation had a beginning, and it surely will have an end. The fulfillment of the various features of the sign clearly indicates that the tribulation must be near. By maintaining your sense of urgency and keeping on the watch, you show that you are keeping up with advancing light and following the leadings of holy spirit.—Mark 13:37.
I included the paragraph that drops plenty of hints that this new idea is evidence of holy spirit at work, and that it represents the light of Bible truth, and that the source is the "faithful and discreet slave." This introduction stands together with the explanation in paragraph 14 that we don't know the length, that it "usually" refers to a given description, and that Jesus "evidently" refers to a version of what it "usually" refers to. The only points that are given with "sureness" are these:
The generation had a beginning It will have an end The fulfillment of various features of the sign indicates that the tribulation must be near Note that it isn't by believing in a potential 'two-lifespan' generation that we are keeping up with advancing light and the leadings of holy spirit, but only explcitly by maintaining a sense of urgency, and keeping on the watch. As Christians we would do this with or without the two-lifespan generation, based on other scriptures, for example:
(2 Timothy 4:1, 2) . . .: 2 Preach the word; be at it urgently in favorable times and difficult times. . .
(Ephesians 5:15, 16) 15 So keep strict watch that how you walk is not as unwise but as wise persons, 16 making the best use of your time, because the days are wicked.
(2 Peter 3:11, 12) . . .Since all these things are thus to be dissolved, what sort of persons ought YOU to be in holy acts of conduct and deeds of godly devotion, 12 awaiting and keeping close in mind the presence of the day of Jehovah. . .