Joe Biden’s opponents focus on his age as something that makes him doddering, confused and ultimately unfit for office. So the great paradox of the 81-year-old’s first term is that he has presided over perhaps the most energetic American government in nearly half a century. He unleashed a surge in spending that briefly slashed the childhood poverty rate in half. He breathed life into a beleaguered union movement. And he produced an industrial policy that aims to reshape the American economy.