On growing up, growing old, and wearing out

There seems to be a vast deal of confusion about the idea of 'growing up.'  Peter Pan and a certain chain of US toy stores are against the very idea.  Eternal youth is the ideal of many Westerners and no few fall into the trap of thinking if they never grow up they'll never get old.  Even Jesus said we must become like children to enter the Kingdom of Heaven.  But what does it all mean?  Are we to remain immature and childish our entire lives to find the fulfillment, happiness, and contentment we desire?  Is immortal youth the key to eternal bliss?  Here's C.S. Lewis on the subject, taken from 'The Last Battle,' the final book in the Narnia series:

"Grown-up, indeed," said the Lady Polly.  "I wish she would grow up.  She wasted all her school time wanting to be the age she is now, and she'll waste all the rest of her life trying to stay that age.  Her whole idea is to race to the silliest time of one's life as quick as she can and then stop there as long as she can."

I read Peter Pan and really did not enjoy it (perhaps that is the whole point of the book?), an immortal youth without responsibility or restraint turns out to be no fun at all.  And as for those who refuse to grow up, living in their parents' basement and never actually taking responsibility for themselves or their actions, they still get old, even if they never mature.  Meanwhile, I look in the mirror and find it hard to believe the birthday that will soon creep upon me, for inside I don't feel half that age and I know at least one octogenarian who is still 26, so it isn't just me.  And as for Jesus, He is in no way implying that we must embrace immaturity with all enthusiasm, rather He's referring to the childlike wonder and open hearts of children in their better moments.  Speaking of which, it is when my son tries to be the most grown-up that he is the least fun to be around: the tantrums, sulking, anger, and frustration over his thwarted attempts to be in control and have his own way.

So what then is the moral?  Remaining stubborn, selfish, and immature like a young and undisciplined child certainly is no fun, for one day we'll wake up and find ourselves 75 years old with the emotional maturity of a twelve year old, having missed out on everything of worth in our lives.  Spending our lives obsessed with a certain physical age only leads to regret and bitterness.  We cannot help growing older physically, but we need not grow old spiritually.  We must certainly 'grow-up' in the sense of becoming mature and wise, but we need not grow old, losing our sense of wonder and innocence and joy, for in doing so, we most certainly shall wear out.  There are many souls that have grown old, even in their third or fourth decade of physical life, whereas, like my octogenarian friend, there are others that burns as brightly in their old age as ever they did in their youth.  This is what Peter Pan actually desired and what Jesus commands.  Society thinks physical youth is all that matters, which makes a certain amount of sense in a materialistic society, but neither Jesus nor Peter Pan are materialists.  Neither should we be, if we truly desire to find Joy.