Can you please explain this to me.............................................................................

The whole reason that people desired the fountain of youth was so they could live forever; never age. This quote is saying that the true fountain, the true key to living forever, is found in your mind. The uniqueness you bring into the world can make an everlasting impression that will influence future generations; like Anne Frank, William Shakespeare, and Aristotle. They are immortalised by the sharing of their mind, the use of the their talents, and the creativity the brought to their world.

In your mind you can imagin you are a youth,You use talents you don't realize you have, (Make believe is one) you would be more creative with things you do,

It means that the personal strengths of our mind and our unique personality, can keep up youthful. I.e. 'your only as old as you think you are'.