The identity of ‘this generation’ has vexed interpreters. Perhaps it is easiest to see a twofold reference, as Jesus has done throughout the discourse. The disciples to whom Jesus is speaking on the Mount of Olives is most naturally ‘this generation’ who sees the events of the destruction of the temple, which shows the applicability of the discourse to A.D. 70. Yet within the context of Jesus’ statements about the coming of the Son of Man at the end of the age, there must be primary applicability to those at the end of the age who see the events surrounding the abomination of desolation occurring. When these signs of the end of the age appear, those waiting for his arrival are to recognize that their redemption is drawing near (Luke 21:28). The generation that sees these things occurring will be the generation that sees the Lord appear.
Craig Evans, in The Gospels and Acts (The Holman Apologetics Commentary on the Bible), offers the following suggestions:
This saying is consistent with the similar prediction in [Mark] 9:1 (‘There are some standing here who will not taste death until they see the kingdom of God come in power’). It is apparent that Jesus’ generation expected to see the fulfillment of the things prophesied in the discourse. And indeed the predictions were partially fulfilled in the events of the first century. That generation saw the destruction of the temple and some of the signs, or at least events that paralleled the signs that will portend the second coming of the Son of Man. But Jesus’ generation did not see the second coming, nor did it see the consummation of the kingdom of God. Jesus spoke of the generation of the last time, not his disciples’ generation. Since ‘this generation’ in Mark refers elsewhere to those who are rebellious and blind (8: 12, 38; cf. Matt 11: 16; 12: 41, 42, 45), it could be used in that sense here, yielding the sense that wickedness will continue until the coming of the Son of Man. Another view is that ‘this generation’ refers to the generation that sees the ‘abomination that causes desolation.’ One further view is that ‘all these things’ refers only to the ‘signs’ of the end rather than to the end itself (Bock 2005, 523).
And finally, John D. Grassmick, in Mark, The Bible Knowledge Commentary, writes:
’Generation’ (genea) can refer to one’s ‘contemporaries,’ all those living at a given time (cf. 8:12, 38; 9:19), or to a group of people descended from a common ancestor (cf. Matt. 23:36). Since the word ‘generation’ is capable of both a narrow and a broad sense, it is preferable in this context (cf. Mark 13:14) to understand in it a double reference incorporating both senses. Thus ‘this generation’ means: (a) the Jews living at Jesus’ time who later saw the destruction of Jerusalem, and (b) the Jews who will be living at the time of the Great Tribulation who will see the end-time events. This accounts best for the accomplishment of ‘all these things’ (cf. vv. 4b, 14–23).
After my study of all these different viewpoints, I find myself leaning toward Brooks, Keener, and Blomberg. However, this is certainly not an issue to be dogmatic about. I have great respect for all these scholars, and it’s quite possible that other interpretations are correct.