Hearing of the death of Milton Friedman, I turned to this biography. What struck me was that Friedman won the Clark Medal, given to the economist under 40 with the most achievements, in 1951. At that time, he had published none of the works for which he is famous, either inside our outside the economics profession. Well, maybe one--the Friedman-Savage utility function (1948). He received the most prestigious award that the profession offers, and in hindsight he had not even gotten started.
It is much easier to think of examples that go the other way--of economists whose most famous work was behind them when they won the award. Looking at the roster of winners, I can think of several who would face uphill tenure battles at a top-five university if only their post-Clark-medal publications were on file. Of the 29 winners, I believe only Friedman, Samuelson, Arrow, Becker, and Stiglitz would have a claim to fame based on work done after they received the Clark Medal.
Middle-aged economists take heart! Friedman was 45 when he produced his book on the permanent income hypothesis, he was 51 when he and Anna Schwartz published A Monetary History of the United States, and he was 55 when he gave his AEA Presidential Address offering the natural rate hypothesis.
Of course, Friedman is best known for outside the economics profession for what he would have called his "normative economics." The late Bernie Saffran used to say, "Milton Friedman and Paul Samuelson teach the same price theory, but only Friedman applies it to policy."