Kant offered many variations on this theme in his lectures on anthropology, claiming that only at forty can we begin to form a correct conception of things because then we have lived through varied situations of life. Before forty, hardly anyone is capable of correct judgments concerning the true value of things. He also emphasized that character is possible only if our inclinations are still sufficiently strong to cause us to take interest in things, but not so strong as to become passions. All this will probably happen at the age of forty. Character requires a ripened understanding. Curiously, Kant also believed that forty marks the year in which the power of memory begins to weaken. Accordingly, we must have collected all the materials for thinking before that year. After forty, "we cannot learn anything new, though we can expand our knowledge." Whatever we will accomplish after forty in intellectual matters is thus a function of the materials we have collected before that time and of the characteristic judgment that develops around forty. It will be the result of our knowledge and our character.
(Manfred Kuehn, Kant: A Biography [Cambridge: Cambridge University Press, 2001], 145 [endnotes omitted])
Keith Burgess-Jackson, J.D., Ph.D. 