Youth is not a time of life - it is a state of mind, it is a temper
of the will, a quality of the imagination, a vigor of the emotions, a
predominance of courage over timidity, of the appetite for
adventure over love of ease.

Nobody grows old by merely living a number of years; people
grow old only by deserting their ideals. Years wrinkle the skin, but
to give up enthusiasm wrinkles the soul. Worry, doubt, self-distrust,
fear and despair - these are the long, long years that bow
the head and turn the growing spirit back to dust.

Whether seventy or sixteen, there is in every being's heart the
love of wonder, the sweet amazement at the stars and starlike
things and thoughts, the undaunted challenge of events, the
unfailing childlike appetite for what next, and the joy and the
game of life.

You are as young as your faith, as old as your doubt; as young
as your self-confidence, as old as your fear, as young as your hope,
as old as your despair.

So long as your heart receives messages of beauty, cheer,
courage, grandeur and power from the earth, from man and from
the Infinite, so long you are young.

When the wires are all down and all the innermost core of your
heart is covered with the snows of pessimism and the ice of
cynicism, then you are grown old indeed and may God have mercy
on your soul.

Source Unknown