John 21:17-23 (NASB)
17  He *said to him the third time, "Simon, son of John, do you love Me?" Peter was grieved because He said to him the third time, "Do you love Me?" And he said to Him, "Lord, You know all things; You know that I love You." Jesus *said to him, "Tend My sheep.
 "Truly, truly, I say to you, when you were younger, you used to gird yourself and walk wherever you wished; but when you grow old, you will stretch out your hands and someone else will gird you, and bring you where you do not wish to go."
19  Now this He said, signifying by what kind of death he would glorify God. And when He had spoken this, He *said to him, "Follow Me!"

20  Peter, turning around, *saw the disciple whom Jesus loved following them; the one who also had leaned back on His bosom at the supper and said, "Lord, who is the one who betrays You?"
21  So Peter seeing him *said to Jesus, "Lord, and what about this man?"
22  Jesus *said to him, "If I want him to remain until I come, what is that to you? You follow Me!"
23  Therefore this saying went out among the brethren that that disciple would not die; yet Jesus did not say to him that he would not die, but only, "If I want him to remain until I come, what is that to you?"

Note that Jesus was not stating that any of the disciples would remain until He comes again to commence His Eternal Kingdom. He was simply stating that what should be of concern to Peter was not whether John would still be alive when Jesus would come again, but for Peter to be focused on following Jesus. So there is no stipulation here that Jesus would come within the time frame of any of the disciples' lifetimes. We are all to be focused on following Jesus.

[(Jn 21:20-23) Bible Knowledge Commentary]:

"21:20-23. Peter, having been informed about God's plan for his life, naturally wondered what the future held for his friend John, the disciple whom Jesus loved. Jesus sharply rebuked Peter for being curious about God's will for another's life: What is that to you? You must follow Me. Some disciples can be easily distracted by unnecessary questions about God's secret will; as a result they neglect God's plainly revealed will. God's plans for Christians vary and His reasons are not often made known. Peter was to commit himself to God's plain commands to him.

John then corrected a faulty inference made by some believers that John would not die. Interestingly Jesus' last words recorded by John in this Gospel refer to His return. Of course, Jesus gave no indication when He would return. The false rumor about Jesus' words to Peter show the possibility of misunderstanding God's promises. Christians must seek to understand God's Word accurately."