Did He mean that generation of His disciples? First, note the future context. Jesus had just said, "So you also, when you see all these things, know that it is near—at the doors!" (Matthew 24:33).
Did His disciples "see all these things" in their generation? Certainly not. They were not in a generation that had the ability to destroy all of mankind. Jesus said of the final, end-time generation, "And unless those days were shortened, no flesh would be saved" (Matthew 24:22). Clearly the weapons of that day were not sufficient to bring this prophecy to pass.
Although Christ's disciples did see wars, famines, plagues and persecutions, and some even lived to see the destruction of Jerusalem several decades later, these were not the total fulfillment of Jesus Christ's words. They were not the prophesied global events that would directly lead to Jesus Christ's return.
So what did Jesus mean when He referred to "this generation"? He could not have meant that current generation of His day. They died without seeing all the events leading to His return. The obvious meaning, then, is that He could only have meant the generation of the last days.
Jesus knew that the course of end-time events would not continue indefinitely. Once they were set in motion, all would occur within the lifetime of one generation. Only one generation will witness all the mounting global conditions described in God's Word. That generation will see the arrival of the specific 3 1⁄2-year period that will mark the end of "this present evil age" and usher in the Kingdom of God.
These events will not pass from one generation to another. Once the prophesied conditions exist, all will be fulfilled—including the return of Jesus Christ—within the lifetime of one generation. Considering the signs Jesus foretold that would mark that time, it appears to be here.