Jehovah's Witnesses teach that the last days started in 1914 because that was the year that the world changed from the start of WW1. The drastic changes from 1914 onward are the sign of the Lord's presence. Those who saw the sign starting in 1914 were the first of a generation that would not pass away before the last days ended. How do JWs define the term "generation" as used here?
I found this on the JW.org website. It is from 2014, and is likely up to date. Prior to this it was taught that some of the group of Christians who were old enough in 1914 to comprehend the significance of the events that proved the messiah's second presence (parousia) had begun in that year would still be alive when the great tribulation starts.
The material below is quoted from the 2014 Watchtower article, "'Let Your Kingdom Come'—But When?"
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.
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. 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.