In the case of Mark and Luke: look at what immediately followed: The Transfiguration. He appeared in His Heavenly Glory to Peter, James and John. It was fulfilled on the 8th day following His promise. It is not so much a prophecy as it is a promise, as He had (and has) the power to do such things. Prophecy has to do with other times, people and occurrences, whereas a promise is made by the individual.
“Generation” From the Rev. George Leo Haydock commentary:
Ver. 34. This generation; i.e. the nation of the Jews shall not cease to exist, until all these things shall be accomplished: thus we see the nation of the Jews still continue, and will certainly continue to the end of the world. T. — Then the cross, which has been a scandal to the Jew, and a stumbling-block to the Gentile, shall appear in the heavens, for the consolation of the good Christian. Hoc signum crucis erit in cœlo, cum Dominus ad judicandum venerit. — If it be to be understood of the destruction of Jerusalem, the sense may be, this race of men now living; if of the last day of judgment, this generation of the faithful, saith Theophylactus, shall be continued: i.e. the Church of Christ, to the end of the world. Wi. — This race, I tell you in very truth, shall not pass away till all this be finally accomplished in the ruin of Jerusalem, the most express figure of the destruction and end of the world. V. — By generation, our Saviour does not mean the people that were in existence at that time, but the faithful of his Church; thus says the psalmist: this is the generation of them that seek the Lord. Ps. xxiii, v. 6. S. Chrys. hom. lxxvii.
Ver. 35. Shall pass away: because they shall be changed at the end of the world into a new heaven and new earth. Ch.