Pendleton Ward, the creator of Adventure Time, has stated in an interview that the Land of Ooo is actually a post-apocalyptic world, the result of a global disaster called the Mushroom War, hence the ruined pieces of modern technology scattered across the land.

Not much is known about the war at the moment, but the inclusion of “Mushroom” in the title is probably referring to a mushroom cloud generated from a nuclear bomb. If so, then it is likely that the Land of Ooo and the creatures who inhabit it were created from the nuclear fallout that irradiated the land. This is further supported by the ruined pieces of modern technology scattered across the land, and the beginning of the opening theme (which briefly shows the remains of nuclear bombs and other weapons). It was confirmed by Pendleton Ward that Marceline and Ice King are survivors of the war due to their supernatural powers.

Strangely, the Land of Ooo also contains technological devices that are far more advanced than what exists today, such as Universal Translators and Holographic Newspapers. It is unknown whether these are based on pre-war technology or are more recent inventions.

An opening shot showing various weapons and technological debris, including grayed land and undetonated nuclear missiles lying around like a Russian SS-13 ICBM, and an arm reaching upward (most likely a zombie) in a tree trunk. There also appears to be the remains of a Tank tread in the center of the shot. The bashed up television is just another remnant from old Earth.

We know that Marceline is 1000 years old, but was born before the Great Mushroom War (as she is a child like vampire in memory of a memory) however, we also know that the Ice King is 1000 years old, and was alive before the great mushroom war. Considering he seems to be somewhere between 20-50 in the tapes from Holly Jolly Secrets, we can presume, using these estimations that the mushroom war occured about 550 years before the episodes are staged.

It is unknown just when the Mushroom War took place. In the episode “Her Parents” it is stated that the Rainicorn-Dog Wars took place over “thousands of years.” However, this does not prove when the Great Mushroom War happened, since the Mushroom War went on in the land that is now Ooo, but the Rainicorn-Dog Wars took place in the Crystal Dimension. Or alternatively Jake may have exaggerated for drama’s sake and simply meant “a very long time” when he said that. In the same episode, Lady Rainicorn’s father, Bob Rainicorn, says that he thought humans were extinct.

In “Memory of a Memory,” Marceline appears as a young girl in a post-apocalyptic environment. In this scene, a burning city in the background suggests the Mushroom War occurred fairly recently, although plants growing in the wreckage of a house could suggest some time has passed since the calamity happened, but it has also been noted in nuclear testing and usage (e.g., Hiroshima) that the radiation encourages plant growth, and days after nuclear bombing occurs plants and flowers start to grow like weeds. Because Marceline has been confirmed to be approximately 1000 years old, this could mean that the Mushroom War took place just under 1000 years before the events of the series. It has been said that the present in Ooo takes place one thousand years in the future.

It could have happened sometime further in the future than the “present day” that the series is airing, i.e. much later than the early 21st century, considering the relics of more advanced technnology they sometimes find.

In Holly Jolly Secrets Part Two, Beemo puts on the last tape which shows Ice King as a young human, who tells the video diary about his fiancé Betty and a crown he wore that tells him secrets (Ice King’s Crown). Throughout the video, he is shown looking more and more like Ice King. The early part of his video diary as a human show him living in what appears to be human civilization, either before or during the war. It is seen through the window that the city is suddenly covered in ice, possibly an the onset of a nuclear winter. This could possibly be the beginning of the Ice Kingdom. It could be determined that Ice King is at least as old as Marceline if both survived the war.

(Source: adventuretime.wikia.com)