The term 'destroyed' might be a bit vague: what I mean by it is not the complete and utter disappearance of an Empire in a blink of an eye, but still a rather abrupt downfall, something lasting maybe 15-20 years (opposed to the hundreds of years some Empires needed to disappear).
I'm going to take "rapid" to mean "could an adult experience their empire rising to its peak, riding that peak, and collapsing"? About 50 years. That's very fast by historical terms.
Next, let's nail down "empire". A thing doesn't have to be called an "empire" to be an empire.
an aggregate of nations or people ruled over by an emperor or other powerful sovereign or government, usually a territory of greater extent than a kingdom
With that in mind, here are the 20th century empires.
The Soviet Empire
The Soviet Empire rose out of the ashes of the Russian Empire (which still exists) gobbling up the Baltic States, most of Eastern Europe in WWII, and edging into Mongolia in the 60s. Some of which was directly incorporated as republics, some left as semi-independent puppet states of the Warsaw Pact.
While it's expansion was effectively halted in the 60s, it was still playing world power in Afghanistan right up to when it fell apart.
In 1989 border controls between the East and West rapidly fell apart and the flood gates were opened. Just two years later would see the Warsaw Pact end and the Soviet Union dissolve into independent countries, and shrunken, but still powerful, Russian Empire.
Yes, a 20 year old in 1944 could be alive to see the empire fall 50 years later in 1992.
The French Colonial Empire
Like many European empires, their expansion period basically ended after WWI in 1919.
It collapsed quickly, starting about 1940 with its defeat in WWII, and ending in 1962 with the independence of Algeria, and it was in a lot of trouble before 1962. So I'd say it qualifies as a quick fall.
1919 to 1962 qualifies.
The British Empire (does not qualify)
Similar to the French, they reached their peak after WWI in 1919 at 4.7 million km2 and immediately began to fall apart. First the Irish Free State and Egypt in 192 and Iraq in 1932.
1931 saw the Statute of Westminster which made certain Dominions (Canada, Australia, New Zealand, Irish Free State, New Foundland, and South Africa) independent nations within a Commonwealth. Despite the British sovereign being in technical control, it's arguable whether they still count as imperial subjects. They could pass their own laws and nullify British ones.
Immediately post WWII saw the independence of India, Palestine, and Pakistan. Malaysia in 1957. And most of Africa by 1968 with Zimbabwe lasting until in 1980. The 1980s saw Canada, New Zealand, and Australia sever their constitutional links to Britain.
The last hurrah for the British Empire would be the Falkland War of 1982. While it ended in British victory, it was costly and illustrated the folly of trying to maintain an empire in the modern world.
1919 to 1982 is a bit of a stretch. And the way it sort of dribbled along for 60 years... I don't think the British Empire qualifies.
Inheriting the ashes of the German Empire after WWI it quite obviously qualifies. Beginning in 1938 with the annexation of Austria, explosively expanding and reaching its height in 1942, and collapsing down to nothing in 1945.
Japan's modern expansion began with the annexation of Korea in 1910 and continued in China and the South Pacific until finally stopped about 1942. By 1945 she was completely stripped of her empire and occupied by a foreign army.
No, it doesn't matter if you still call yourself "Emperor". An emperor with no empire, no "aggregate of nations or people" to rule over, is just a king.
American Empire (does not qualify)
While it can be argued that The American Empire starts as soon as colonial settlers arrive and start annexing native lands, it wasn't really as an aggregate of nations. The US didn't rule over an aggregate of nations, they annexed their lands, wiped out their people, and replaced them with their own.
The American Empire begins at the end of the Spanish-American war in 1898. The US gained Cuba, Puerto Rico, the Phillipines, Guam, and a smattering of Caribbean and Pacific islands. It has no intention of turning these into states. The US suddenly and awkwardly finds itself in control of an overseas empire.
The US would continue to intervene in Caribbean, Central and South American, and Pacific nations through WWI and the 1930s with some excuse or another. If you elected a government the US didn't like, the US might invade. If you threatened US business interest, the US might invade. For example, Haiti was occupied from 1914 to 1934.
Most of the US' colonies and protectorates would be made independent post WWII with the exception of Puerto Rico and some small islands. Like the British, the American Empire would undergo a morphing from direct rule to a sort of quasi-Empire of mutual interest. Sometimes bullying, sometimes protecting.
One can argue it still has not fallen, nor stopped expanding, nor was ever an empire. So I don't count the American Empire.