"'Truly I tell you, this generation will certainly not pass away until all these things have happened. Heaven and earth will pass away, but my words will never pass away.'" NIV translation
All of those last days events have not happened but all of the apostles have died; so, what's up? The Hebrew understanding of generation is not a straight line like we think but is a circle of events and time. There was a circle from the days of Adam until the destruction of Noah's day. There was a circle from the days after the flood to the destruction of the temple. The generation that Jesus was talking about was the time from the giving of the Holy Spirit to the final judgment.
"'But about that day or hour no one knows, not even the angels in heaven, nor the Son, but only the Father.'" NIV translation
When this latest generation (circle) will end is only known to the Father. As we have seen, the only remaining clue that we have is the sign that Jesus said to look for which is the "abomination that causes desolation". That will be the start of the Great Tribulation.
"'Be on guard! Be alert! You do not know when that time will come. It's like a man going away: He leaves his house and puts his servants in charge, each with their assigned task, and tells the one at the door to keep watch. Therefore keep watch because you do not know when the owner of the house will come back - whether in the evening, or at midnight, or when the rooster crows, or at dawn. If he comes suddenly, do not let him find you sleeping. What I say to you, I say to everyone:"Watch!"'" NIV translation
Jesus reminded His disciples that we are like servants that have been given a job while the master is gone. We are to be about that task until He returns. We will know of His imminent arrival when we see the sign that was given. We all (those in Christ) have a task to do before we meet Jesus. If that task is done before He returns, then, we will go to meet Him. If it is not, then, we should be working at it when He arrives.