Emerson's quote, "It was high counsel that I once heard given to a young person, 'always do what you are afraid to do,'" could be understood as follows.
First, Emerson's use of "high counsel" indicates that he believes this advice to be extremely valuable. We can also surmise that it is especially important that the advice be best taken while a person is still young.
One would expect that he admires this advice, perhaps because he is now older and did not do the same--or did so and finds the advice to be precise and true. The old adage "too soon old, too late smart" comes to mind. Maybe he has learned by experience, which is often the case in Emerson's writing as he so often looks within and shares his observations in his writings.
When we are young, the world is fresh and all things are possible. We can confront the future as if it were a blank page, and we can write whatever we wish on that page. Our choices will shape our lives and we will never again have that opportunity to freely follow the path of our choosing. Later in life, when we have financial and/or familial responsibilities, taking a new path is possible, but comes with the considerations attached to our responsibilities.
Youth also comes with marvelous hopes and dreams, and the energy, the innocence and the passion to work tirelessly to reach those lofty heights. When we are young, we so often will believe that what may seem to be impossible to others (whose vision has--over the years--been tainted by disappointment and skepticism), are just within reach. Life takes its toll and often as we grow older, we don't have the belief in ourselves to reach for the golden ring.
With regard to Emerson's use of "fear," this speaks to a young person's self-doubts. The world is quick to quash youthful dreams when those dreams have been unattainable to older generations. We could argue that Emerson sees the unique qualities in each person and holds with the belief that if we, as young people, can quiet the voices of doubt and cynicism around us, we can experience living to the fullest, enjoy a life met head-on with exuberance, and achieve all of which we are capable. If not, we may look back some day with regret, a wasted emotion--something that cannot be changed.
From Emerson's perspective, one can assume that he encourages us while we are young, perhaps even young at heart, to meet life with a belief in what is possible, without fear. This is easier when we are young, but perhaps Emerson would agree that if we keep the energy and innocence of youth within, perhaps we all can achieve great things if we move forward without fear.