Youth is not a period of time.

 It is a state of mind, 
a result of the will, 
a quality of the imagination, 
a victory of courage over timidity, 
of the taste of adventure 
over the love of comfort. 

A man doesn't grow old 
because he has lived a certain number of years. 
A man grows old when he deserts his ideal. 

The years may wrinkle his skin, 
but deserting his ideal wrinkles his soul. 
Preoccupations, fears, doubts, and despair 
are the enemies, which slowly bow us 
toward earth and turn us into dust before death. 

You will remain young as long as 
you are open to what is beautiful, 
good and great; 
receptive to the messages of 
other men and women, 
of nature and of God. 

If one day you should become bitter, 
pessimistic and gnawed by despair, 
may God have mercy on your old man's soul.

General Douglas MacArthur