From the chapter on habit, here's James on the importance of developing productive habits early on:
We all intend when young to be all that may become a man before the destroyer cuts us down. We wish and expect to enjoy poetry always, to grow more and more intelligent about pictures and music...We mean all this in youth, I say; and yet in how many middle-aged men and women is such an honest and sanguine expectation fulfilled? Surely, in comparatively few, and the laws of habit show us why. Some interest in each of these things arises in everybody at the proper age, but if not persistently fed with the appropriate matter, instead of growing into a powerful and necessary habit, it atrophies and dies, choked by the rival interests to which the daily food is given.Or, more pithily, are you going to be an interesting middle-aged person or are you going to be someone whose tastes are frozen in time at age 22?
We say abstractly: "I mean to enjoy poetry, and to absorb a lot of it, of course. I fully intend to keep up my love of music, to read the books that shall give new turns to the thought of my time..." But we do not attack these things concretely, and we do not begin today. We forget that every good that is worth possessing must be paid for in strokes of daily effort. We postpone and postpone, until those smiling possibilities are dead. Whereas ten minutes a day of poetry...and an hour or two a week at music, pictures, or philosophy, provided we began now and suffered no remission, would infallibly give us in due time the fulness of all we desire. By neglecting the necessary concrete labor, by sparing ourselves the little daily tax, we are positively digging the graves of our higher possibilities.