This question had a dramatic influence on Albert Schweitzer when he was studying New Testament theology. Jesus said, "This generation will not pass away until all of these things come to pass . . . You will not go over all the cities of Israel until all of these things come to pass . . . Some of you will not taste death until all of these things come to pass."
Schweitzer looked at those passages, and he thought of them as obvious cases where Jesus blew it, where Jesus expected his return in the first century. Schweitzer saw this expectation of the early return of Jesus in early writings of Paul. Then there was an adjustment in the later writings of the Bible to account for the great disappointment that Jesus didn't show up in that first generation. That's been a matter of great consternation for many people.
Jesus didn't say, "Some of you aren't going to die until I come back." He said, "Some of you will not taste death until all of these things come to pass." The difficulty lies in the structure of the language. The disciples are asking Jesus about the establishment of the kingdom. Jesus talks about two distinct issues. He talks about what obviously involved the destruction of Jerusalem when he said that the temple would be destroyed. Then at the end of the Olivet discourse, he talks about his return on clouds of glory.
Some of the best New Testament scholarship that I've seen is on the meaning of the Greek words translated "all of these things." An excellent case can be made that when Jesus used that phrase, "these things" of which he was speaking pertained to the destruction of the temple and of Jerusalem. It's amazing that Jesus of Nazareth clearly and undeniably predicted one of the most important historical events in Jewish history before it took place. This wasn't just a vague Nostradamus or Oracle of Delphi type of future prediction; Jesus vividly predicted the fall of Jerusalem and the destruction of the temple, which indeed took place in A.D. 70, while many of his disciples were still alive. It was also before the missionary outreach had reached all of the cities of Israel and before that generation had, in fact, passed away. Those cataclysmic events that Jesus had predicted on the Mount of Olives did, indeed, take place in the first century.