1874-1931: "this generation" means those living who saw the Arrival of Christ in 1874, the Enthronement 3.5 years later (in 1878), and the end of the world in 1914 (later changed to a "spiritual ending") and the fulfillment during this time (1874-1914) of all the sign given in Matthew 24, Mark 13, and Luke 21. This understanding began to blur into the "new" understanding in 1929, four years after the failed "Millions" predictions of returning Ancient Worthies.
1929-1941: "this generation" means those living who saw the tumultuous "Time of Trouble" begin. Although this time was not nailed to a certain date the older Bible Students/JWs took it to mean 1874 while the younger set were being groomed to focus more on 1914.
1941-1954: "this generation" is specified to refer to those who were witnesses to the Enthronement of Christ in heaven in 1914 (1878 having been dropped ages before) and the associated commencement of the Time of Great Trouble on earth. This generation would not die until Armageddon had come, as the final stage of the Great Tribulation.
1954-1995: "this generation" is still tied to the witnessing of Christ's enthronement in 1914, but there is some waffling throughout these years on whether this is associated in any way with the Great Tribulation until, eventually, the Great Tribulation was pushed forward as a yet future event.
1995-2007: "this generation" refers to those who witness the wicked, twisted generation of mankind upon whom final destruction will come. Its commencement is no longer associated with a fixed time, nor does it have any rational terminus—despite the fact that a rational terminus is implicit in the phrase "shall not pass away" which follows.
2007: apparently refers to the remnant of the 144,000 yet remaining, including the recent GB additions who are in their 50s and others who are similarly regarded as validly anointed by reason of having passed the litmus test of the sitting GB conjecturing it so. This means, as long as the GB can keep formally recognizing new anointed (via appointment to the GB) they can keep this dogma afloat. Why this doesn't smack every single JW as parlor trick mental manipulation I have no idea. But, my brain was massaged a few good rounds when I was in, too, so I can't really be too critical.