These words of Jesus follow the triple question and response dialogue between Jesus and Peter in which Jesus asks, "Simon, son of John, do you love me?" The questions seem to match the triple denial of Jesus by Peter in the high priest's courtyard. The evangelist comments that the words quoted above about growing old signify how Peter would die. But they strike home to any of us who are growing older (I'm 82!). There are the inevitable challenges of physical health - diminishment of strength and energy as well as body parts wearing out. But there is also the tension between the "mind" and the body. Each seems to have its own agenda.
I am always both amused and challenged by the line from Isaiah 65:20 in the third part where he is prophesying about the Israel to come after the Babylonian exile: "He will die but a youth who lives to be a hundred years old!" A common expression one hears nowadays is "Age is only a number!" And friends send a lot of "elderhumor" that is both funny and painful! But Jesus strikes a universal note in his admonition to Peter. My hope is that Jesus is the one who will dress me and lead me where I really want to go. (But not right away, please?) AMEN