This teaching is not about the length of a generation but what this particular generation sees that identifies them as the generation Jesus is speaking of.
How is Mt.24:34 explained by Jesus? That "this generation will by no means pass away till all these things take place."
One needs to understand the larger picture before they can put the details together.
This teaching begins in Matt 24:3 the disciples came to Him privately, saying, "Tell us, when will these things be? And what will be the sign of Your coming, and of the end of the age?"
This what Jesus is about to answer throughout this discourse.
V. 4 – deception
v.5 they will come in his name to deceive
v.6 hear of wars and rumors of war
v. 7 it involves nations and kingdoms, famines, pestilences, and earthquakes take place all over the world.
v.8 – this is the beginning of sorrows
v.9 They will gather believers to be killed, the nations of the world will hate Christians
v.10 many will fall away, stumble, a great betrayal will take place among Christians
v.11 many false prophets will rise up and deceive many ( false prophets among the believers to deceive them)
v.12 lawlessness will be the norm, the love of many (believers) will grow cold.
v.13 the person who endures to the end (of this time period) shall be saved. Those who continue in their faith until Jesus returns (applicable for all generations of believers)
v.14 the gospel of the kingdom will be preached in all the world as a witness to all the nations ( we read of this taking place in Rev. 7), and then the end (of the age) will come.
v.1 5 they will see the 'abomination of desolation,' spoken of by Daniel the prophet, standing in the holy place" (which means the Temple in Israel must be built, it is the Holy place)
v.16 Judeans are to flee to the mountains.
v.17 the abomination in the temple will have an immediate effect. They will not have time to get their belongings in their household.
v.18 nor those in the field to get their clothes. Their life is in danger to do this.
v.19 it will be especially difficult for those who are pregnant and to those who are nursing babies in those days.
v.20 if this takes place in winter or on the Sabbath it will also be difficult.
v.21 this is when the great tribulation commences, it will be a unique time as Jesus tells us not since the beginning of the world until this time; it will never be repeated. (Luke 21:24)
v.22 unless those days were shortened, no flesh would be saved; so many people will be killed that if it continued no human would be left. But for the elect's sake those days will be shortened. It is for those who have salvation that Jesus does not let it continue and intervenes.
v.23 -24 they are to be aware in this time period of anyone says to you, 'Look, here is the Christ!' or 'There!' do not believe it. For false christs and false prophets will rise and show great signs and wonders to deceive, if possible, even the elect. This is a time of deception unlike any in all of history, they will have power to do miracles and signs and wonders.
v.25 See, I have told you beforehand. (Mark 13:23 is more explicit, all things are told). we have no excuse
v.26 Then saying 'Look, He is in the desert!' do not go out; or 'Look, He is in the inner rooms!' do not believe it. This is one of the particular rumors in this time period, where he is appearing, manifesting at these places.
v.27 the coming of the Son of Man will be come as quick as it takes lightning to occur.
v.28 For wherever the carcass is, there the eagles (birds of prey) will be gathered together.
Jesus inserts a point of his coming (Armageddon). The birds of prey will eat the carcasses of those who killed that participated in the the battle of Armageddon (Gen. 15:12).
v.29 "Immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light; the stars will fall from heaven, and the powers of the heavens will be shaken. This is what is described as the tribulation commences at the 6th seal Rev 6:12-13 and the sun became black as sackcloth of hair, and the moon became like blood. And the stars of heaven fell to the earth, ( this continues in Rev. 8:12
v. 30 the sign of the Son of Man will appear in heaven, and then all the tribes of the earth will mourn. The sign is what they will see—which is the Son of Man coming on the clouds of heaven with power and great glory (clouds often referring to God’s glory He ascended in a cloud, Acts 1:9. And will return in like manner, Acts 1:11) This will be especially vivid as the backdrop is when the sun is darkened.
v.31 And He will send His angels with a great sound of a trumpet, and they will gather together His elect from the four winds, from one end of heaven to the other. Here Jesus rescues the elect that are being killed. This is not the rapture that is instantaneous for our transformation, angels are not involved in it (except one to blow the great trumpet.) this trumpet announces the gathering for the angels to save Israel and others.
v. 32 Jesus now give the parable from the fig tree: When its branch has already become tender and puts forth leaves, you know that summer is near. The fig tree has its fruit grow along with its leaves.
v. 33 So you also, when you see all these things, know that it is near--at the doors!
Jesus return is about to take place.
v. 34 Assuredly, I say to you, this generation will by no means pass away till all these things take place.
Again in context v.33-35 Here Jesus summarizes , So you also, when you see all these things, know that it is near--at the doors! Assuredly, I say to you, this generation will by no means pass away till all these things take place.
All these things that he said before are to identify “the generation” that he is addressing. it is this generation that sees all these things taking place that will see his return.