Youth means a temperamental predominance of courage over timidity of the appetite for adventure over the love of ease. This often exists in a man of 60 more than a boy of 20. Nobody grows old by the number of years. We grow old by deserting our ideals.
Years may wrinkle in the skin, but to give up enthusiasm wrinkles the soul. Worry, fear, self-distrust bows the heart and turns the spirit back to dust. Whether 60 or 16, there is in every human being's heart the lure of wonder, the unfailing childlike appetite of what's next and the joy of the game of living. In the center of your heart and my heart there is a wireless station : so long as it receives messages of beauty, hope, cheer, courage and power from men and from the Infinite, so long are you young.
When the aerials are down, and your spirit is covered with snows of cynicism and the ice of pessimism, then you are grown old, even at 20, but as long as your aerials are up, to catch the waves of optimism, there is hope you may die young at 80."
- Samuel Ullman
Reading the above quote, do you feel inspired? I do! And this quote is what I have on my table at work. I don't have to elaborate further, everything is mentioned in it about what real youth is.