This exciting prediction by Christ climaxes His great prophetic discourse on the Mount of Olives. He had given His disciples the signs they had requested, then discussed the coming great tribulation and finally His glorious return in the clouds of heaven. And "all these things" were to be fulfilled before "this generation" would pass away.
And what generation would that be? Many commentators have taken it as the Jewish "race," but that would be redundant, since many other passages had already promised that the nation of Israel would never pass away (Jeremiah 31:37-40; etc.). Furthermore, the Greek word for "generation" (genea) is never used elsewhere for any meaning but that of a particular age generation. A similar word genos sometimes means "stock" or "kind," but never genea.
Thus, the generation which Christ was predicting probably meant the generation that would see the events He had prophesied. "When ye shall see all these things," He said, "know that it is near, even at the doors" (Matthew 24:33).
And what are some of "these things"? World wars, accompanied and followed by "earthquakes in divers places," as well as "famines, and pestilences" (v. 7), worldwide spread of the gospel witness (v. 14), many false Christs and false prophets (v. 24), widespread wickedness and spiritual indifference as in the days of Noah (vv. 37-39), and the budding of the fig tree, Israel (v. 32).
The word for "this" in verse 34 is the demonstrative adjective, so Christ seems to be referring to "that" generation which sees "these things begin to come to pass." That generation will see all these things fulfilled! To that generation He says: "Lift up your heads; for your redemption draweth nigh" (Luke 21:28). HMM
http://www.icr.org/article/6324/