“Then Peter, turning about, seeth the disciple whom Jesus loved following; which also leaned on His breast at supper, and said, ‘Lord, which is he that betrayeth thee?’ Peter seeing him saith to Jesus, ‘Lord, and what shall this man do?’
Jesus saith unto him, ‘If I will that he tarry till I come, what is that to thee? Follow thou me.’
Then went this saying abroad among the brethren, that that disciple should not die: yet Jesus said not unto him, ‘He shall not die;’ but, ‘If I will that he tarry till I come, what is that to thee?'” John 21:20-23  (KJB)

Remember, if you will, that at the trial of Jesus, and Peter standing with the crowd outside denied our Lord three times.  I am positive that Peter grieved over that when he heard that rooster crow, and it hit him like a rock, and he remembered what the Lord said.

Jesus is giving Peter some assurance, and reassurance in much of this chapter.  Giving him renewed faith, and strength.

The Lord has revealed to Peter what his life is going to be like for Him; and so he asked the Lord the question concerning John, “What shall this man do?” He receives the Lord’s answer, “If I will that he tarry till I come, what is that to thee? Follow thou me.”

Peter was not given words concerning John’s life, his calling, or his last day on earth. Peter was given Peter’s life.

The same is true of all Christians.  We are not to be anxious about another’s life, their calling, neither their last day or days.  Not that we do not care for them; but that we are not using them to excuse our own weaknesses, sins, and faults.

Jesus’s answer to Peter could be said to be “You, follow Me”.  That is what the Lord’s answer to me is; and so for all who call Him Lord and Savior.