This generation. Twelve times over in Matthew's gospel the term "this generation" is used.  Matt. 3:7; 11:16; 12:34, 39, 41, 42, 45; 16:4; 17:17; 23:33, 36; 24:34. Half of those are found in ch.11-12. It refers to the moral class of individuals among the nations of Israel that have been the rejecters of God's testimony throughout the nation's history. We have this pattern all though prophecy: judgment upon the mass, and the deliverance of a remnant. We know that this is a class of persons by the context in which the expression occurs. For example, in Matt. 23 it speaks of "this generation" saying; "That upon you may come all the righteous blood shed upon the earth, from the blood of righteous Abel unto the blood of Zacharias son of Barachias, whom ye slew between the temple and the altar." It isn't that God will attribute the sins of some to others, but that judgment would finally fall on that class in light of their sins all down through the centuries. The Lord describes them vividly:

Typically, a generation is a matter of years (20 yrs. or so), a lifetime at most. But the meaning of a "generation" in prophecy is moral rather than chronological. For example; in Psalm 12:7, "Thou shalt keep them, O Lord; Thou shalt preserve them from this generation for ever." The term "this generation" is coupled with "for ever", which in the Old Testament does not denote eternity, but a great length of time. And so it is in the prophetic scriptures. This is important because Preterists will use verses like Matt. 24:34 to insist that the fulfillment of Daniel's seventieth week occurred within one literal generation.