Have you ever been to Washington, D.C.? Have you walked on the Mall, seeing the Washington Monument, the Capitol Building, the Lincoln Memorial, and the Smithsonian buildings lining the way? Imagine hearing someone say, in full seriousness, that a day was coming when not one of those marble blocks would be left upon another.
That is always the feeling I get when Jesus talks with His disciples about the destruction of Jerusalem. (I am not predicting the destruction of our nation's capital - just trying to get a sense of what Jesus' words must have felt like.) It's no wonder that hearing Him talk like this prompted their question, "Uh, when will this be?"
For Jerusalem, it's destruction certainly did happen before that generation had passed away, within about 40 years' time. Jesus talks about the events that will happen then, as well as the development of the Church until that time, and also about the end, which will take place when the "time of the Gentiles are fulfilled."
Of course, nobody knows when the end will come. But Jesus' message is the same to each and every generation - be ready.