From: Luke 21:29-33
Discourse on the Destruction of Jerusalem
and the End of the World (Continuation)
 And He (Jesus) told them a parable: "Look at the fig tree, and all the trees;  as soon as they come out in leaf, you see for yourselves and know that the summer is already near.  So also, when you see these things taking place, you know that the Kingdom of God is near.  Truly, I say to you, this generation will not pass away till all has taken place.  Heaven and earth will pass away, but My words will not pass away."
31. The Kingdom of God, announced by John the Baptist (cf. Matthew 3:2) and described by our Lord in so many parables (cf. Matthew 13; Luke 13:18-20), is already present among the Apostles (Luke 17:20-21), but it is not yet fully manifest. Jesus here describes what it will be like when the Kingdom comes in all its fullness, and He invites us to pray for this very event in the Our Father: "Thy Kingdom come." "The Kingdom of God, which had its beginnings here on earth in the Church of Christ, is not of this world, whose form is passing, and its authentic development cannot be measured by the progress of civilization, of science and of technology. The true growth of the Kingdom of God consists in an ever deepening knowledge of the unfathomable riches of Christ, in an ever stronger hope in eternal blessings, in an ever more fervent response to the love of God, and in an ever more generous acceptance of grace and holiness by men" ("Creed of the People of God", 27). At the end of the world everything will be subjected to Christ and God will reign for ever more (cf. 1 Corinthians 15:24, 28).
32. Everything referring to the destruction of Jerusalem was fulfilled some forty years after our Lord's death--which meant that Jesus' contemporaries would be able to verify the truth of this prophecy. But the destruction of Jerusalem is a symbol of the end of the world; therefore, it can be said that the generation to which our Lord refers did see the end of the world, in a symbolic way. This verse can also be taken to refer to the generation of believers, that is, not just the particular generation of those Jesus was addressing (cf. note on Matthew 24:32-35).
[The note on Matthew 24:32-35 states:
32-35. Seeing in the destruction of Jerusalem a symbol of the end of the world, St. John Chrysostom applies to it this parable of the fig tree: "Here He also foretells a spiritual spring and a calm which, after the storm of the present life, the righteous will experience; whereas for sinners there will be a winter after the spring they have had [...]. But this was not the only reason why He put before them the parable of the fig tree, to tell them of the interval before His coming; He wanted to show them that His word would assuredly come true. As sure as the coming of spring is the coming of the Son of Man" ("Hom. on St. Matthew", 77).
"This generation": this verse is a clear example of what we say in the note on Matthew 24:1 about the destruction of Jerusalem being itself a symbol. "This generation" refers firstly to the people alive at the time of the destruction of Jerusalem. But, since that event is symbolic of the end of the world, we can say with St. John Chrysostom that "the Lord was speaking not only of the generation then living, but also of the generation of the believers; for He knows that a generation is distinguished not only by time but also by its mode of religious worship and practice: this is what the Psalmist means when he says that `such is the generation of those who seek Him' (Psalm 24:6)" ("Hom. on St. Matthew", 77).]
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.