The first sign of this great shift came in Russia earlier this year when Dmitry Medvedev, at 44 just two years older than Johnson, succeeded Vladimir Putin as president. The crucial sign will come in the United States later this year, where uniquely all three generations are battling for the presidency.
As well as Sen. Hillary Clinton, the classic baby boomer, and the young upstart Sen. Barack Obama, the classic generation X-er (and son of a restless and globe-trotting baby-boomer mother), Sen. John McCain represents the Silent Generation.
Born in the Great Depression of the 1930s to a career Navy officer and thus a member of what Americans now call the greatest generation for their wartime feats, McCain falls into that gap before the coming of the baby boomers that is known as the Silents. They were the ones who came of age in the 1950s, that period of American conformity and the anti-Communist witch hunts of the McCarthy era.
It has always been unfair to call this generation the Silents, when they produced the beatniks, rock 'n' roll, Elvis Presley and Margaret Thatcher, and launched the civil rights movement in the United States. And few of them were less silent than the rowdy, rebellious and trouble-making fighter pilot McCain.
Only two of those three will contend for the presidency in November, and it seems at the moment that Clinton the baby boomer is likely to be the first casualty, leaving the old and young men to fight it out. Should McCain win, he will be one of the last standard-bearers of his generation on the world stage. Should Obama win, he will be one of the first of his generation, but many more are waiting in the wings.
Johnson's victory in London's election was important not simply because running one of the world's greatest cities is bound to be a high-profile job but also because of its portent for the next British government. The current leader of the Conservative opposition is David Cameron, Boris' old schoolmate at Eton, grandest of England's private schools. A fellow member of Generation X, Cameron will challenge the baby-boomer Prime Minister Gordon Brown at the next general elections. Boris's victory bodes well for another baby-boomer defeat.
It does not seem that long ago since Bill Clinton emerged as the first baby boomer to take the world stage in 1992, with Tony Blair following in 1997 and Gerhard Schroeder then emerging in Germany in 1998 and Junichiro Koizumi in Japan in 2001.
The boomers swept all before them, but not one can claim to have been a great political success. Clinton left under the impeachment cloud, Blair under the grim shadow of the Iraq war; Schroeder in defeat and Koizumi in the underachievement of his reform plan. The baby-boom revolutionaries who took power with such high hopes fell sadly, and in many cases shamefacedly short.
The baby boomers, traditionally spoiled by indulgent parents and raised under the "breast-feeing on demand" rule of the child-raising guru Dr. Benjamin Spock, always felt entitled.
So Clinton left his presidency under the scandal of that rash of pardons, and his post-presidency has been marked by a singular devotion to raising funds, some of them for his presidential library and foundation now raising questions that are damaging his wife's election campaign. Schroeder has given up the chancellorship for a controversial job with Russia's Gazprom energy giant, helping secure a pipeline under the Baltic Sea that will allow Russia to turn off the power in Poland and Ukraine at will, without inconveniencing Western European customers.
By contrast, the silent generation did not do badly at all. It includes Mikhail Gorbachev and Boris Yeltsin, who between them dismantled the wretched Soviet Empire with remarkably little bloodshed. Nelson Mandela did the same for South Africa. Among their fellow stars of the silent generation are Germany's Helmut Kohl and France's Jacques Chirac, China's Jiang Zemin and India's Manmohan Singh. All in all, McCain is in distinguished company.
One definition of the Generation X-ers is that they were "born with the Beatles," that is after their first appearance on the U.S. charts in 1963. It means that they grew up with some knowledge of the excesses of the '60s and the difficulties caused by the OPEC price rise and the great inflation of the 1970s. They also learned of the power of political leadership to turn things round, with Ronald Reagan in the United States and Thatcher in Britain.
Whether or not Obama wins the presidency and Cameron becomes prime minister, the Generation X-ers are coming. Their rise is inevitable, and after the disappointments of the baby boomers that could yet be redeemed with the one last chance of Clinton, the X-ers should be an interesting bunch.