What is the meaning behind John 21:18 - 19?

^18 "Very truly I tell you, when you were younger you dressed yourself and went where you wanted; but when you are old you will stretch out your hands, and someone else will dress you and lead you where you do not want to go." ^19 Jesus said this to indicate the kind of death by which Peter would glorify God. Then he said to him, "Follow me!"

I was just thinking about these verses earlier today, and realized that they seem quite vague and cryptic. As I have usually heard, these seem to be a reference to the traditional martyrdom narrative of Peter, being crucified upside down, but (validity of that narrative aside) when I read the verse, there's nothing in them that really indicates that it is referring to a martyr's death, let alone a crucifixion.

What is meant by the stretching out of hands, being dressed by someone else, and being led to undesirable places? Is the kind of death Jesus indicates merely that of old age?