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I am anticipating the Labour leadership election result with mixed feelings. For the result may undermine my first rule of leadership elections - that the winner is usually the youngest candidate.

Between them, the three main British parties have held 19 ordinary leadership elections since the war. By that I mean proper elections, either amongst MPs or a wider electorate. And I don't include contests which involve a sitting leader simply being challenged, such as John Major v John Redwood in 1995.

Nor do I include the two contests where someone was elected unopposed - Michael Howard (2003) and Gordon Brown (2007).

Four of the six Conservative elections saw the youngest contender win - John Major (1990), William Hague (1997), Iain Duncan Smith (2001), and David Cameron (2005).

The exceptions were Edward Heath (1965) and Margaret Thatcher (1975), though Thatcher was younger than her main opponent Willie Whitelaw.

Labour scores four out of seven - Hugh Gaitskell (1955), Harold Wilson (1963), Neil Kinnock (1983) and Tony Blair (1994). The exceptions were James Callaghan (1976) (though he was younger than his main rival Michael Foot), Michael Foot (1980), and John Smith (1992).

The Liberals and Liberal Democrats have also elected the youngest candidate in four of their six elections - Jeremy Thorpe (1967), David Steel (1976), Charles Kennedy (1999), and Nick Clegg (2007). The exceptions were Paddy Ashdown (1988) and Ming Campbell (2006).

That's 63 per cent.

63 candidates fought the 19 contests, so if the results were random in terms of age, one would have expected the youngest to win 28 per cent.

So being the youngest contender makes it more than twice as likely that you will win.

And some of the youthful winners - Hague, IDS and Kennedy - triumphed over four older rivals.

If you include Roy Jenkins' defeat of David Owen for the leadership of the Social Democratic Party (SDP) in 1982, the probability falls to 60 per cent.

In the last 20 years, since John Major's election in 1990, the trend has been even more pronounced - seven elections out of nine, including all four Conservative leadership contests.

That's a rate of 77.8 per cent. The two exceptions - the Scottish lawyer close friends John Smith and Ming Campbell - each lasted less than two years in office.

These results would seem to suggest that British parties are increasingly putting a premium on youth and freshness when picking their leaders.

So where does this leave us with the current Labour contest?

Andy Burnham is the youngest contender, being just a fortnight younger than Ed Miliband. Burnham isn't going to win, of course, but Ed Miliband may well do so, and he is four and a half years younger than his brother, David.

So the youth and freshness factor might still prove important in the Labour election, and may be decisive, although the strict result itself on Saturday 25 September 2010 will almost certainly undermine my rule.

NB: My previous version of this blog mistakenly omitted Michael Foot, who was not a young winner. These figures above are now the revised ones. The trends are still clear, especially since 1990.