The fact British prime ministers have been getting younger when they assume office is obvious, but there has been almost no comment on the consequences of this development.

The relative youth of recent prime ministers may be the slow-acting result of the student revolts that erupted in 1968 across the western democracies including Britain. The ensuing so-called “counter culture” put the emphasis on youth and usurped the hegemony traditionally accorded to their elders and betters. Respect was to give way to celebrity, which by its very nature gives short shrift to seniority.

One of the consequences has been that it is now almost impossible to envisage the election of a leader of a political party aged over forty-five. Ming Campbell’s short spell as leader of the Liberal Democrats ended prematurely due to the incessant cartoons depicting him relying for support on a Zimmer frame. It was easier for David Steel to endure being depicted in David Owen’s breast pocket or for Nick Clegg to tolerate being characterised constantly as David Cameron’s fag than to rise above being tagged with crutches and a Zimmer frame.

In an attempt to reverse the popular disdain for the over-fifties, I toyed with a Lords PQ in the last Parliament asking if the government would hold a referendum on whether Messrs Cameron, Osborne, Clegg and the Miliband brothers should be allowed to wear long trousers? The Clerks would not have allowed it as being frivolous but it would have struck a blow against the dominant cult of youth that is now, literally, the ruling factor in British politics.  So much for grey power!  The creation of the UK Youth Parliament in 1998 was gratuitous – merely adding insult to injury: the generational coup had already been secured.

But it is not the relative youth of recent heads of government that matters – after all, William Pitt holds the record that is unlikely to be broken – so much as the post is now effectively a mid-career one. Pitt died young but the greater longevity of his latest successors creates new problems.

For most of the twentieth century, British prime ministers moved into No 10 after working in the outside world as a lawyer, trade unionist or whatever then, on entering the Commons, undertaking long parliamentary apprenticeships as backbenchers before ascending the ministerial hierarchy and reaching the top. On demitting office, they would move into semi-retirement and write their political memoirs with a warm sense of gratification that they had risen to the top of the ‘greasy pole’; such was the traditional last three or four of the “seven ages” of political man/woman.

Now, the pattern has changed entirely. True, recent prime ministers may rush out their memoirs quicker than their more elderly predecessors but that is to catch the market before the memories of the reading public fade. These books also serve to provide an early foundation for their “legacy” – a word which has now taken on an entirely new connotation. And, in any case, there is every certainty that such autobiographies will have later editions encompassing the post-parliamentary careers of their authors.

In retrospect, the aspirations of earlier prime ministerial cohorts seem relatively modest. Burning ambition was satiated by having presided over the political system for a period. Jim Callaghan preferred the certainty of another eight months in office rather than risk being ejected sooner if he called a General Election in the autumn of 1978, even though Labour might have polled better had he opted for the earlier date. His decision vividly illustrates the mind-set that prevailed but which was about to change radically with the advent of a breed of younger prime ministers.

The new generation’s ambitions would not be satisfied with the highest office – it would merely spur it on to achieving success in subsequent careers. These invariably have a major international dimension because, like bishops, they must vacate their diocese on stepping down. Thus, joining the boards of global conglomerates, taking on specific (but vacuous) diplomatic tasks, world-wide lecture tours, or setting up one’s own international consultancies to advise other nation- states or corporations, occupy ex-prime ministers.

Such second careers cover the transition from middle age to ultimate retirement, enable the building- up of considerable personal fortunes and, of great psychological importance, nurture “the legacy” that will be further elaborated in the revised editions of the memoirs – ghosted or not!  

Despite the extreme pressures of work that beset all modern prime ministers, the youthful generation might be forgiven for not always giving work their fullest attention precisely because of the need to plan for later. In this, of course, they are no different from others of their own age who are overweeningly ambitious in other walks of life - but then, the latter don’t have their fingers on the nuclear button!

The Americans thought they had anticipated the problem. Those US Presidents, who avoided assassination, received generous pensions on vacating the White House and garnered huge sums to set up their memorial libraries. In this way, they were well provided for and given material aid to secure their historical legacy as best they may. This provision, of course, did not constrain Bill Clinton and is unlikely to inhibit Barack Obama in the pursuit of further fame and fortune. Having been head of government looks good on a CV.

Vladimir Putin’s Russian variant of coping with the problem of mid-career employment is to contrive to stay in office for as long as it suits him. 

Trevor Smith is a septuagenarian LibDem working peer who in his youth was chair of the Union of Liberal Students.