A stint as a federal prosecutor in a busy district has long been a ticket into Big Law’s partnership ranks for ambitious young lawyers. The average age for assistant U.S. attorneys is a relatively tender 47, according to Yale Law School figures.

At 60, Bruce Keller is bucking that trend. The former head of Debevoise & Plimpton’s intellectual property litigation group, Keller quietly left the firm this summer to become an assistant U.S. attorney for the District of New Jersey, where he’ll serve in the office’s appeals division.