Really? Thought it was first mentioned in 2010 and there is mention of it in the January 15th 2014 Study Edition of the WT...
The original mention is here:
Watchtower (Study Edition), April 15th, 2010 Issue, Pages 27-29: "This magazine has long been used by “the faithful and discreet slave” as the primary channel for dispensing increased light. ... 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 indicate 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."
The 2014 WT has this:
15 In his detailed prophecy about the conclusion of this system of things, Jesus said: “ This generation will by no means pass away until all these things happen. ” (Read Matthew 24:33-35.) We understand that in mentioning “this generation,” Jesus was referring to two groups of anointed Christians. The first group was on hand in 1914, and they readily discerned the sign of Christ’s presence in that year. Those who made up this group were not merely alive in 1914, but they were spirit-anointed as sons of God in or before that year. —Rom. 8:14-17. 16
The second group included in “this generation” are anointed contemporaries of the first group. They were not simply alive during the lifetime of those in the first group, but they were anointed with holy spirit during the time that those of the first group were still on earth. (implying that this second group are made up of pretty old people)
Thus, not every anointed person today is included in “this generation” of whom Jesus spoke. Today, those in this second group are themselves advancing in years. Yet, Jesus’ words at Matthew 24:34 give us confidence that at least some of “this generation will by no means pass away” before seeing the start of the great tribulation.
This should add to our conviction that little time remains before the King of God’s Kingdom acts to destroy the wicked and usher in a righteous new world.—2 Pet. 3:13.