One thing that might be surprising: the Soviet gerontocracy was old but only by Soviet standards. Brezhnev was 75 when he died, Yuri Andropov was 69, Konstantin Chernenko was 73, Alexei Kosygin was 76, Dmitri Ustinov was 76, the list could go on. These Soviet leaders were of the same age or even younger than Reagan (who turned 70 just after his 1981 inauguration), and quite a bit younger than the last two US presidents have been while in office. Putin himself is almost 70.

So what was the deal with the Soviet gerontocracy of the late 1970s and early 1980s? Two factors: one, these Soviet leaders were almost universally in horrible health. This is from a combination of excessive drinking, smoking and poor eating as well as overwork which tended to exacerbate issues like heart disease and nervous system problems, with the result that many of them suffered repeated heart attacks and strokes while remaining in office.

Which leads to the second issue - they remained in office. The reason behind this is largely a reaction to Stalin (who died at 74 by the way) - many of the Soviet leaders until Gorbachev were part of what was known as the "Class of 1939" - they were the young whipper-snappers with no formative memory of pre-1917 Russia who were promoted to positions of party authority in the late 1930s by Stalin.

They mostly replaced the "Old Bolsheviks", who had been members of the Party before the Revolution (and whom Stalin had seen as rivals). They often replaced their replacements. They sometimes replaced the replacements of their replacements. This was all because of the Great Purges of the late 1930s, when Stalin had anyone he saw as a potential political rival to his authority arrested, sometimes jailed in gulags, but often shot (the number of executed reached about a million, and were mostly Party members).

So while the "Class of '39" was seen by Stalin as the bright young future of Soviet power, they came of age in the circumstances of massive upheaval and death in the party, and then had to hold on through the Second World War. After Stalin's death, and the subsequent turmoil of Khrushchev's reforms, most of this generation of leaders wanted things to slow way down, and hence "Stability of the Cadres" became a policy under Brezhnev. Essentially this meant that no life-and-death struggles for power would be countenanced any more - there would be a collective leadership which would bicker among themselves for more senior spots, but otherwise everyone would basically remain in some sort of senior leadership role. It ultimately was an overreaction, pushing the next generation of potential leaders (like Gorbachev, who came of age in the 1960s) into a long pattern of waiting, but it was in some ways understandable - it was seen as a means to avoid the repeats of the very real traumas of personnel "turnover" in the Stalin years.