An impression John Paul II always made on me, even in his last days, was that he was essentially a young man. I've not encountered anyone else who has made such an impression, and I've pondered off and on for years what could be the source of this impression.
The youthfulness of JPII is different than, say, the boyishness of Robert Redford or Brad Pitt. It expresses something deeper, as though JPII expressed throughout his life the essence of what it means to be young . Perhaps it would be more accurate to say that JPII expressed the virtue of youth, or the virtue that is best expressed through youth. The most apt comparison I can think of is the wizard Gandalf in The Lord of the Rings. Gandalf expresses the essence or virtue of what it means to be old; in Gandalf's case, that virtue is obviously wisdom. Wisdom is the prerogative of age, and the wise man is essentially old, as Socrates is already old when he appears in Plato's dialogs. And so when we imagine Gandalf, we imagine him as eternally old. We can't imagine Gandalf a young man.
The virtue of hope is essentially expressed through youth, and I think it is because JPII so thoroughly embodied hope that he struck me as essentially young. Hope expresses confidence with respect to the future (although not in the future; we hope in God for the future). With respect to the future we are all young, so hope is the virtue of the young. If one has hope, even if he is 90 years old and physically decrepit, he is young.
I wonder if our culture's worship of youth is an expression of despair; we are without hope and so long for the one period of life when hope, or the appearance of hope, was a possibility. But as Kierkegaard pointed out, such "hope" is not genuine but an aesthetic suspension of the self in possibility. As long as we have the possibility of becoming many things, but have not yet become any, the illusion of hope is before us. But as soon as any possibility is actualized, it must be at the expense of other possibilities ("opportunity cost", although Kierkegaard didn't call it that) and the pseudo-hope of youth disappears. So the aesthetic self chooses to remain in the realm of possibility, not really becoming anything at all, and so retains for itself the illusion of hope; but it is only an illusion and nothing but a yet deeper expression of despair.
JPII appears youthful even in old age because he was the apostle of hope. True hope is not intoxicated with possibility, nor does it suspend itself in the possible without ever becoming actual. True hope is anticipation of what is to come on the far side of actuality; it is longing for the consummation of possibility in actuality. John Paul II was one of the most actual men who ever lived. I'm tempted to use the word commitment with respect to him, but commitment has a voluntaristic tint to it that reflects more our modern subjectivist orientation than what JPII was about. Submission may be a better word because it has an objective orientation; we submit to things that are outside of and greater than ourselves; we "commit" to "values" the significance of which is grounded in our own will. True marriage is a submission rather than a commitment. John Paul II's life was animated by a deep submission to God, and so was grounded in true hope for what will come on the far side of that submission.