A man grows old when he deserts his ideal.

"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 for 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. "(Gen. Douglas MacArthur)