Cubans are accustomed to being informed of their leader Fidel Castro's thoughts usually in the form of long-winded speeches or rambling articles. But a letter from Castro read out on Cuban state television two days ago contained something they had never heard before. Towards the end of his missive, Mr Castro told his audience that he had a duty not to "cling to office" or obstruct the rise of "younger people". This is the first time the octogenarian Cuban leader has so much as hinted at retirement since he seized power in the revolution of 1959.

Mr Castro did not indicate when he might step down. But this letter is clearly part of an attempt to prepare Cubans for a transition of power. Elections are due to be held next month to choose the Cuban national assembly, which will then select the president of the Cuban council of state. It seems that, for the first time, someone other than the ailing Mr Castro could be chosen.

In truth, this moment has been expected for quite some time. In July 2006, Mr Castro temporarily handed power to his younger brother, Raul, after having surgery for a mysterious illness. He has been conspicuously absent from state occasions ever since.

Thoughts inevitably now turn to the succession. Mr Castro's reference in his letter to "younger people" would seem to hint that 76-year-old Raul is not the certainty that many imagined. Mr Castro's protg, Felipe Perez Roque, and the Vice-President, Carlos Lage, are both in the frame.

There can be little doubt that Cuba needs to look to a younger generation for leadership. Despite its impressive health system, the economy is in serious trouble. Living standards for most Cubans have been in decline since the collapse of the Soviet Union in 1991. Tourism and oil subsidies from Venezuela have helped keep the show on the road in recent years but only just.

Cuba's new leader will need to open up the economy and bring the country back in from the cold internationally. The departure of George Bush from the White House in early 2009 is an opportunity for the US to be (for once) a constructive force in Cuba. The new US administration should begin by lifting the 40-year-old trade embargo. But internal political reform in Cuba is just as essential. One-party rule and the jailing of dissidents must end.

Mr Castro has defied the odds over almost five decades of uninterrupted rule. But he has finally run out of time. It is time for the curtain of history to descend on the extraordinary Castro era.