Jesus said "some who are standing here" would see Him in His glory. Peter, John and James were those men and they did indeed see Him. They got a foretaste of that reality.
The second group (the generation) to see Him will be when Jesus comes back at the Tribulation's end to defeat the Antichrist and the false prophet and to set up His kingdom upon the earth, the people alive when all those things that were foretold to happen in the end times are fulfilled will see Him in His glory and kingdom. It will all happen within a set number of years once the ball gets rolling. Remember when Jesus spoke about seeing things "begin to take place":
These things will all take place within a lifespan. Some of these things, I believe, will even be witnessed by believers before the Rapture happens and will be how we can know it will be soon look up and we await His catching us up to be with Him with joyful anticipation. We won't know the day or the hour, but we can know the times and the seasons. Soon...
I am not sure if what Peter, James and John saw is valid as it speaks about his “departure,” which would predates his return.
In Mark 14:61-62, Jesus declares that the high priest who delivered him over to the Romans for execution will see the Son of Man coming on the clouds of heaven.
Then in John 21:22-23, Jesus says that the Apostle John will remain alive until His return.
This is a fascinating Study. I highly suggest a study on “New Heavens and New Earth” God promised to never destroy the Earth and all on it after the Flood. Reason suggests that if God cannot lie; then the destruction of the Earth and Heaven, MUST refer to something different.
It is paradox’s like these that must be solved before one can even come close to seeing the Scriptures for what they really are. The Holy Bible is the most amazing book ever printed. It’s not a hand book on how to live your life, though many will use for that and be happy. It is not history book and should not be taken literally 100% To do so (and many do) is to gloss over to real meaning (try a study on the number 1000) The Holy Scriptures were written by a number of different authors, each with a slightly different view and is riddled with apparent contradictions.
Is it a book of mysteries, a book of puzzles; perhaps. I think it as a book of tapestry with multiple stories of different colored threads. It’s up to the read to weave these threads into an amazing Tapestry of Truth.
God is Love, God is Truth and God is Spirit; this must be the basis for all understanding.