You will not taste death
“But I tell you truly, there are some standing here who will not taste death until they see the kingdom of God.” (Luke 9:27) What does this verse mean, coming as it does almost immediately after Peter confesses that Jesus is “The Christ of God” (v. 20)? The verse is difficult because the immediate context gives no clue about what “the kingdom of God” refers to exactly. There is a crisis—we know that much—because the words let us know that this thing that has been expected for centuries is now about to happen—even within the lifetime of the hearers. But what does it mean to “see the kingdom of God”?
As we read further, we find that the very next thing that happens is the transfiguration, which serves as a divine confirmation on Peter's confession in 9:20 (“[you are the] Christ of God”). Then there is the humbling incident of the demon that the disciples, for all the privilege of their position as followers of this Christ, have been unable to dislodge. Both these incidents are, I would say, linked to Christ's confession and the challenge that Jesus has laid down to forsake all to follow him.
Then in Luke 9:51, “[Jesus] set his face to go to Jerusalem”. This is a major statement by Luke, and its importance is reinforced just a couple of verses later by repetition (v. 53: “[T]he people did not receive him, because his face was set toward Jerusalem ”). In fact, this statement governs the next 10 chapters or so because, from now on, Jesus is indeed heading towards Jerusalem where he will meet his death.
He will die, yes, but all the others who heard Jesus saying “[T]here are some standing here who will not taste death until they see the kingdom of God” will still be alive (with the possible exception of Judas. See Matt 27:5-8 and also Acts 1:18—neither of which tell you when he died). So the first possibility for the timing of the coming of the kingdom is Jesus' death in Jerusalem.
This would also explain the urgency of the mission of the disciples then recorded in Luke 10:1-24. Although, now that I think of it, when the disciples return from this mission, perhaps the kingdom has already come. Note Luke 10:18: “I saw Satan fall like lightning from heaven“, which is followed up almost immediately by Jesus saying, “Blessed are the eyes that see what you see! For I tell you that many prophets and kings desired to see what you see, and did not see it, and to hear what you hear, and did not hear it.” (Luke 10:23-24)
Anyway, maybe this question of timing is like watching a car accident unfold. When is the accident happening? When the car hurtles through the red light at 80 km/hr? When the other car fails to notice the first car? When the first car, realizing what has happened, begins to swerve and brake? At the point of collision? Or when both cars have come to a screaming standstill? You could argue that all these events are part of the one accident. In a similar way, you could argue that the miracles and teaching of the Christ—the transfiguration; the mission of the 72; the trip to Jerusalem; the rejection by the elders, chief priests and scribes; the killing of Jesus, and his resurrection on the third day—all these are part of the single coming of the kingdom of God. It has come, and now we, the messengers of God, are preaching it to the nations.