Writers take a while to attain full power

From Thomas Babington Macaulay’s “A Speech Delivered In The Committee of the House Of Commons On The 6th Of April 1842” (Prime Palaver #4: 1 September 2001):

It is the law of our nature that the mind shall attain its full power by slow degrees; and this is especially true of the most vigorous minds. Young men, no doubt, have often produced works of great merit; but it would be impossible to name any writer of the first order whose juvenile performances were his best. That all the most valuable books of history, of philology, of physical and metaphysical science, of divinity, of political economy, have been produced by men of mature years will hardly be disputed. The case may not be quite so clear as respects works of the imagination. And yet I know no work of the imagination of the very highest class that was ever, in any age or country, produced by a man under thirty-five. Whatever powers a youth may have received from nature, it is impossible that his taste and judgment can be ripe, that his mind can be richly stored with images, that he can have observed the vicissitudes of life, that he can have studied the nicer shades of character. How, as Marmontel very sensibly said, is a person to paint portraits who has never seen faces? On the whole, I believe that I may, without fear of contradiction, affirm this, that of the good books now extant in the world more than nineteen-twentieths were published after the writers had attained the age of forty.