...it's always best to go to the scholars -
Gill's Exposition of the Entire Bible
Verily I say unto you..... This is a strong asseveration, Christ puts his "Amen" to it; declaring it to be a certain truth, which may firmly be believed:
there be some standing here; meaning either his disciples, or some of the audience; for it is clear from Mark 8:34 that the people were called unto him with his disciples, when he said these words:
which shall not taste of death: that is, shall not die; a phrase frequently used by the Jewish doctors: they say (y),
"All the children of the world, , "taste the taste of death".''
That is, die:
till they see the Son of man coming in his kingdom; which is not to be understood of his personal coming in his kingdom in the last day, when he will judge quick and dead; for it cannot be thought, that any then present should live to that time, but all tasted of death long before, as they have done; for the story of John's being alive, and to live till then, is fabulous, and grounded on a mistake which John himself has rectified at the close of his Gospel: nor of the glorious transfiguration of Christ, the account of which immediately follows; when he was seen by Peter, James, and John, persons now present; for that, at most, was but an emblem and a pledge of his future glory: rather, of the appearance of his kingdom, in greater glory and power, upon his resurrection from the dead, and his ascension to heaven; when the Spirit was poured down in an extraordinary manner, and the Gospel was preached all over the world; was confirmed by signs and wonders, and made effectual to the conversion and salvation of many souls; which many then present lived to see, and were concerned in: though it seems chiefly to have regard to his coming, to show his regal power and authority in the destruction of the Jews; when those his enemies that would not he should reign over them, were ordered to be brought and slain before him; and this the Apostle John, for one, lived to be a witness of.