In a nutshell:
The Watchtower tried to explain the word ‘generation’ in Matthew 24:34 by referencing Exodus 1:6 to support overlapping groups.
However, Matthew was written in Greek and Exodus was written in Hebrew. To understand how the writer of Matthew understood the word generation, we must find out how the writer used the word throughout the book, and in its proper context.
In Matthew 23, in the denunciation of the Pharisees, we find Jesus saying in verse 36, “Verily I say unto you, All these things shall come upon this generation.” What things? The things revolving around the destruction of Jerusalem.
Matthew 24 is a continuation of Matthew 23. Any separation between these chapters, as we all know, is artificial. The narrative of chapter 23 logically continues in chapter 24. The entire context of chapter 24 is the destruction of Jerusalem.
Even verses 2-3 need be read without division: And He said to them, “Do you not see all these things? Truly I say to you, not one stone here will be left upon another, which will not be torn down.” As He was sitting on the Mount of Olives, the disciples came to Him privately, saying, “Tell us, when will these things happen, and what will be the sign of Your coming, and of the end of the age?”
Therefore, it is in this context, that of the things revolving around the destruction of Jerusalem, that verse 34 says, “Truly I say to you, this generation will not pass away until all these things take place.”
Matthew 24 is set about 30 to 33 CE, while the destruction of Jerusalem was in 70. About 40 years. A generation.
Matthew 1:1-17 totally devastates any idea of an overlapping generation. It lists the generations from Abraham to Jesus. Verse 17 says, “So all the generations from Abraham to David are fourteen generations; from David to the deportation to Babylon, fourteen generations; and from the deportation to Babylon to the Messiah, fourteen generations.”
Abraham is one generation, Isaac is the second, Jacob is the third, Judah, the fourth and so on. If we do the math, for instance from the deportation to Babylon to the birth of Jesus, it works out to be about roughly 40 years. The word generation is devoid of any sense of multiple overlapping groups.