There is plenty of disagreement on what Jesus meant when He said, “Truly, I say to you, this generation will not pass away until all these things take place” (Matthew 24:34, Mark 13:30, Luke 21:32). He was making a reference, of course, to all that He had said prior to this statement. This included prophecies about war, earthquakes, famine, false prophets, persecution in the synagogues, the spreading of the gospel to all nations, the surrounding of Jerusalem and Judea by a foreign army, a time of unparalleled distress, His coming in the clouds with great power and glory, the arrival of His kingdom, and redemption for His people.
The careful reader will take note that Jesus was prompted to make these predictions in response to His disciples’ question about when the temple would be destroyed, and what signs would point forward to that event (Matthew 24:1-3, Mark 13:1-4, Luke 21:5-7). It was this question that led to His discourse, known as the Olivet Discourse.
Mark and Luke, in their accounts, only show the disciples asking this one question about the temple. Matthew shows them asking a three-part question or, some would say, three separate questions:  When will the temple fall?  What will be the sign of Your coming?  What will be the sign of the end of the age? Those who believe in fulfilled eschatology maintain that these events are synonymous, while those who regard this discourse as unfulfilled often say that the last two belong to our future.
When it comes, then, to the timing of this prophecy’s fulfillment, there is debate at both ends of the Olivet Discourse:
 Did Jesus set out to answer just one question? Or did He set out to answer three questions, and therefore He may have prophesied about two different time periods?
 When Jesus said “this generation,” did He mean His own generation? Or did He have in mind, as some are fond of saying, a future generation that would begin to see those signs take place all at once? Did He even perhaps imply a dual fulfillment, a partial fulfillment in His own generation, and an ultimate fulfillment in the far distant future?
We will set aside the first question for the rest of this article, and focus on the second question, for Jesus is shown to say precisely the same thing in all three accounts: “…this generation will not pass away until all these things take place.”
At the very beginning of the New Testament, we get a good idea of how Matthew defined the word “generation.” Consider his genealogy listing from Abraham to Jesus:
“So all the generations from Abraham to David are fourteen generations, from David until the captivity in Babylon are fourteen generations, and from the captivity in Babylon until the Christ are fourteen generations” (Matthew 1:17).
We learn from Matthew that 14 generations passed from the Babylonian captivity until the time of Jesus. We also know that Babylon took Judah captive in 586 BC. Putting these two pieces together, we can calculate that each of these 14 generations was about 42 years in length (586 divided by 14). [For more details on this, see my study on Matthew 24:29-34 / Mark 13:24-30 / Luke 21:25-32.]
This being the case, let us ask if the temple was destroyed, as Jesus predicted, within one generation of that prediction, i.e. within approximately 42 years. It was! Jesus was crucified in or around 30 AD, and the temple was destroyed by the Roman armies in 70 AD, that is, 40 years later.
The Olivet Discourse was not the first time that Jesus had used the expression, “this generation.” In all His other uses of this phrase, it’s more than evident that He meant His own generation. Consider the following examples:
“To what can I compare this generation? They are like children sitting in the marketplaces and calling out to others: ‘We played the pipe for you,and you did not dance; we sang a dirge,and you did not mourn.’” (Matthew 11:16-17)
“The men of Nineveh will stand up at the judgment with this generation and condemn it; for they repented at the preaching of Jonah, and now something greater than Jonah is here. The Queen of the South will rise at the judgment with this generation and condemn it; for she came from the ends of the earth to listen to Solomon’s wisdom, and now something greater than Solomon is here.” (Matthew 12:41-42; see also Luke 11:29-32)
“Then it goes and takes with it seven other spirits more wicked than itself, and they go in and live there. And the final condition of that person is worse than the first. That is how it will be with this wicked generation.” (Matthew 12:45)
“You unbelieving and perverse generation,” Jesus replied, “how long shall I stay with you? How long shall I put up with you? Bring the boy here to me.” (Matthew 17:17)
And so upon you will come all the righteous blood that has been shed on earth, from the blood of righteous Abel to the blood of Zechariah son of Berekiah, whom you murdered between the temple and the altar.Truly I tell you, all this will come on this generation.” (Matthew 23:35-36)
He sighed deeply and said, “Why does this generation ask for a sign? Truly I tell you, no sign will be given to it.” (Mark 8:12)
Jesus went on to say, “To what, then, can I compare the people of this generation? What are they like?” (Luke 7:31)
“But first He must suffer many things and be rejected by this generation.” (Luke 17:25)
Time and space limitations will not allow us to examine all the signs that Jesus predicted would take place before “this generation” would pass away, although I believe that  a study comparing Scripture with Scripture and  a study of 1st century history will show that they did take place before Jesus’ own generation passed away. As an example of the first point, Luke says that Jews “from every nation under heaven” heard the mighty works of God proclaimed in their own languages (Acts 2:5-11), and the apostle Paul was emphatic that the gospel was preached all over the (known) world in his lifetime (Romans 1:8, Romans 16:25-26, Colossians 1:5-6, and Colossians 1:23).
However, as helpful as these confirmations may be, they are not even necessary for our understanding of Jesus’ words, “…this generation will not pass away until all these things take place.” The generation that would pass away had to see, not some, but all of those things take place. There’s only one generation that witnessed the fall of the temple, and that was Jesus’ own generation. This will never happen again, and this point alone indicates that “all these things” could not possibly take place in our own generation or in the future. Remember also that the earthquakes, wars, the invasion of Judea and Jerusalem, the great distress, and all the other signs were to take place, not after the temple fell, but before the temple would fall (Matthew 24:1-3, Mark 13:1-4, Luke 21:5-7).
To the people of Jesus’ time, we who live in the 21st century would be regarded as “that generation.” To those of us living in the 21st century, the people of Jesus’ time would also be regarded as “that generation.” Jesus didn’t use that phrase, however. The people of Jesus’ time would have understood their own generation as “this generation,” just as they did when He uttered this phrase in Matthew 11:16-17; Matt. 12:41-42; Matt. 12:45; Matt. 17:17; Matt. 23:35-36; Mark 8:12; Luke 7:31; Luke 11:29-32; and Luke 17:25.
All of our studies on the Olivet Discourse can be seen here, including a verse-by-verse parallel study of Matthew 24-25, Mark 13, and Luke 21.