Some will not taste of death for an age
“Verily, verily, I say unto you, If a man keep my saying, he shall never [ou mē eis aiōn, ‘not for an age’] see [theōreō] death. Then said the Jews unto him, Now we know that thou hast a devil. Abraham is dead, and the prophets; and thou sayest, If a man keep my saying, he shall never [ou mē eis aiōn] taste [geuomai] of death” (John 8:51-2).
Some will not taste of death “till” Christ comes
“Verily I say unto you, There be some standing here, which shall not taste [geuomai] of death, till [heōs] they see the Son of man coming in his kingdom” (Matthew 16:28).
“And he said unto them, Verily I say unto you, That there be some of them that stand here, which shall not taste [geuomai] of death, till [heōs] they have seen the kingdom of God come with power” (Mark 9:1).
“But I tell you of a truth, there be some standing here, which shall not taste [geuomai] of death, till [heōs] they see the kingdom of God” (Luke 9:27).
“There are those of the rising generation who shall not taste death till Christ comes” (Joseph Smith, HC 5:336).
As posted here, sometimes “till” can mean “until and after.” The following quotations would suggest this is the case with the above quotations.
Some will not taste of death, but will be changed from mortality to immortality
“And he said unto them: Behold, I know your thoughts, and ye have desired the thing which John, my beloved, who was with me in my ministry, before that I was lifted up by the Jews, desired of me. Therefore, more blessed are ye, for ye shall never taste of death; but ye shall live to behold all the doings of the Father unto the children of men, even until all things shall be fulfilled according to the will of the Father, when I shall come in my glory with the powers of heaven. And ye shall never endure the pains of death; but when I shall come in my glory ye shall be changed in the twinkling of an eye from mortality to immortality; and then shall ye be blessed in the kingdom of my Father. . . . And now behold, as I spake concerning those whom the Lord hath chosen, yea, even three who were caught up into the heavens, that I knew not whether they were cleansed from mortality to immortality—But behold, since I wrote, I have inquired of the Lord, and he hath made it manifest unto me that there must needs be a change wrought upon their bodies, or else it needs be that they must taste of death; Therefore, that they might not taste of death there was a change wrought upon their bodies, that they might not suffer pain nor sorrow save it were for the sins of the world. Now this change was not equal to that which shall take place at the last day; but there was a change wrought upon them, insomuch that Satan could have no power over them, that he could not tempt them; and they were sanctified in the flesh, that they were holy, and that the powers of the earth could not hold them. And in this state they were to remain until the judgment day of Christ; and at that day they were to receive a greater change, and to be received into the kingdom of the Father to go no more out, but to dwell with God eternally in the heavens” (3 Nephi 28:6-8, 36-40).
“And it was by faith that the three disciples obtained a promise that they should not taste of death; and they obtained not the promise until after their faith” (Ether 12:17).
“After this vision had closed, another great and glorious vision burst upon us; for Elijah the prophet, who was taken to heaven without tasting death, stood before us” (D&C 110:13).
Some will not taste of death because it will be sweet to them
“And it shall come to pass that those that die in me shall not taste of death, for it shall be sweet unto them; And they that die not in me, wo unto them, for their death is bitter” (D&C 42:46-7). These people experience physical death, but they do not taste the bitterness of death because they die in Christ.