What made the generation-switch hard in Soviet Union?

One reason was the heavy fighting in the Soviet Union during World War II, that took a heavier toll of the later cohort of the so-called World War II generation, than in most other countries. The group born between 1915-1924 in the United States supplied most of its fighting men. But the Soviets drafted (and lost) men as young 17 in 1945 (born 1928). China fielded far fewer men than the Soviets, and while the casualties in the two countries were probably comparable, China's population was about three times larger than the Soviet's/

Andropov, born 1914, was the youngest of the "aging" group born earlier in the 20th century. The "next" leader, Gorbachev was born in 1931, because the generation between them had been (more than) decimated in the war.

On the other hand, America had three Presidents, Kennedy, Carter, and Bush Sr. born from the 1917-1924 cohort, and even China had two leaders, Hua Kuo Feng, and Zhang Ze Min born in the 1920s.

Source: Wikipedia biographies of the various leaders.

THE stereotypes of political life almost always cast young leaders as harbingers of change and old leaders as promoters of continuity. Yet when one looks at the Soviet Union and China, the casting director must have broken the traditional mold. Russia under the 56-year-old Mikhail Gorbachev is changing slowly and hesitantly. China under the 82-year-old Deng Xiaoping is changing rapidly and courageously.

While researching this topic, I found an excellent article on this very subject: Russia and China: the young vs. old leaders. Two communist systems - but how different. It contrasts the rapid pace of reform in China compared to that of USSR, in 1987. If you want to read an answer from a real expert from the time, as opposed to amateurs on the internet, just go there.

There were indeed many factors making modernisation slow in USSR, but fast in China post-Mao. While it's true that younger generations are generally more reform-minded than the old, in this case it would be a minor factor - indeed, as the article notes, the 56-year-young Gorbachev is slower to reform than the elderly 82-year-old Deng. There were many reform-minded people in the older generations, even in USSR.

• The political foundations of the Chinese Communist Party are much more diverse than that of the Soviets'. In the establishment of USSR, Russia voluntarily withdrew from a world war and fought a civil one, leading to one small faction - the Bolsheviks - claiming ultimate power. The People's Republic was unlike this; already being embroiled in a costly civil war, China had an invasion foist upon them, leading to the very strange bedfellows, Communists and Chiang's proto-Fascist KMT. The party was a mix of old-school Communists, Mao's peasantry, modernists, intellectuals, patriots, and enemies of Chiang.
• USSR was much more heterogeneous than China, both politically and ethnically. Ethnic Russians made up only 51% of the population, whereas ethnic Han were 96%. USSR spanned 100 nationalities, China 1. Perhaps most important, USSR ruled over an empire over Eastern Europe, a politically unstable entity that was constantly on the verge of breaking apart, especially if liberalisation was too rapid in USSR. Reform too rapidly, and things like the invasion of Czechoslovakia happens. Whereas Deng was the unquestioned paramount leader, who happened to be a reformer, and could do many things as he pleased.
• Stalin's purges eliminated many political rivals, including reformists. You could say the same for the Cultural Revolution, where many so-called "Rightists" were purged, but the affair spun out of Mao's control, who actually wanted to save people like Deng. While De-Stalinisation was slow, reform-minded factions led by Deng outmaneuvered Mao's successor Hua Guofeng (who was a moderate reformer himself) within a short 2 years and gained control.
• WWII would also be a factor, keeping Stalin firmly in power despite the destabilising purges. Post-Mao, there were no crises in China to keep Maoists in power.
• Much should be said about Deng himself, who was unique as a pragmatic reformer. Learning from the inimitable Lee Kuan Yew, Deng set up Special Economic Zones in certain small towns and cities, allowing free enterprise and foreign investment. This was unthinkable in a country where bartering was still a crime.
• The geopolitical realities were different. USSR was a global superpower with expansionist goals; its military was key to existence, its enemies legion and formidable. China was (is?) a regional power with few aggressive plans (with notable exceptions like Taiwan) and fewer enemies. It could afford to place the military last in its modernisation plans. and perform massive demobilisations.

This question is factually silly. Deng Xiaoping was in the same generation as Mao Zedong. Deng Xiaoping was present in Guangxi and Jiangxi, and he was present for the long march, the anti-Japanese war, and the fight against the GMD. He was born in 1904, and was just 9 years younger than Mao Zedong.

Officially, he was only in office between 1981 and 1987, but it is well known that he retained plenty of power after his "retirement." During that time, he groomed new leadership in younger ranks, thus developing a system for transfer of power to new leaders. It is notable that China at this time was not at risk of foreign invasion or subversion; China was not engaged in the "Cold War." (wisely, in my opinion)

The USSR, at the height of the cold war after Stalin's death, had no such luxury. The timing is similar: Stalin died 32 years after the founding of the Soviet Union, and Deng took power 32 years after the PRC was founded. However, the PRC didn't have to endure WWII or any other major foreign engagement.

It seems that the Soviet Union was to busy with the Cold War to develop smooth succession plans the way the PRC did.

The correct question would be "Why the Chinese manage to change their leaders smoothly?" Lifetime leadership is typical for Communist dictators, not only in Soviet Union. Cuba, Yugoslavia, East Germany, Roumania,... you can continue yourself.

The two leaders of Communist Russia who stepped down, Khrushchev and Gorbachev, stepped down as a result of coup d'etat. (Of course the second coup d'etat was sort of legal, they just dissolved the Soviet Union of which Gorbachev was president. But still this was a secretly planned conspiracy, not a normal legal process).

Speaking of China, it is of course an exception among the communist dictatorships, and not only in this respect, but in many other respects. Perhaps the next stage of evolution:-)

I don't think they were particularly unusual in that regard. At the time of Andropov, all the other nations with permanent membership on UN Security Council (USA, England, France, and China) were lead by WWII-era politicians. All were WWII veterans, with the obvious exception of Margaret Thatcher.

The USA wouldn't get its first post WWII-generation president for another decade. If anything, Gorbachev stepped into a situation where he was a relative youngster on the world stage.