How old does an event have to be to be considered 'History'?

IF it moves.........it will become History. Whether it is a Nanosecond or The Origin of the Cosmos, ALL things have been, are and will always be subordinate to the Laws of time......specifically, historical time. It could be your own biography, to the longevity of the Parthenon; it could be the historical lifespan of the town you live in, to the formation of Earth itself; it could be an antique car you wish to drive, to the Age of the Dinosaurs.....ALL of these things, were part of a historical reality-(whether it was recent or archaic).

So let's say you're studying Contemporary History, that is to say, the History of the past 25 years or so-(From 1992, to the present-day), is that truly History? Yes, it is History, though it is, "Contemporary History", meaning that it is a historical period that is of our generation which is still chronologically distant from our present time. The imploding collapse of the Soviet Union in 1991, was a cataclysmic historical event, the introduction of the Internet as a public utility during the mid 1990's, was a historic time and January 1st, 2000, was also a significant event in world history-(specifically, the Western world, marking the starting point of the 3rd millennium AD/CE). These events and episodes over the past 25 years may seem like, "Current Events" and are not often viewed as historically consequential. On the contrary, these above mentioned events are very much etched into the tapestry of world history and, over time, will be better appreciated and correctly understood to have been a part of Greater History.

I am an American and am basically answering for an American point of view. The authors below believe that some non-American cultures have adopted an American timetable, post World War II, but are not sure of that.

In their book, "Generations," William Strauss and Neil Howe (S&H) characterize "big events" in American history set approximately 80 years apart: 1781, 1861, 1941, (with the next one "scheduled" for the early 2020s). Their point was that "history" was what took place before the last "big event," (World War II), and the "modern era" was what took place after.

That explains why in American history classes, even today, people were "only taught about events that happened until one or two years after World War Two. The Cold War was covered in Geography classes more like it was something new..." More to the point, that was the way things were when I was growing up in the 1960s, when World War II was only 20 years in the past.

Note that in America, "pre-war" and "post-war" refer to World War II, while "ante-bellum" and "post-bellum" refer to the Civil War.

So "recent" events will be pushed into history after the next "big event." What happens thereafter will then be the "modern" era.

Remember that documents pertinent to the historical analysis are classified with release dates ranging from 5 years to 150 years. As these documents are slowly released, and historians have the time to read and analyze them, the understanding of historical events can sometimes change in significant ways.

For instance, the knowledge of the ENIGMA code-breaking was only declassified in the 1970's, and documents leading to the belief that Dieppe was an attempted snitch of ENIGMA key-codes only around the year 2000.

In this sense, an event can only be thought of as fully historical and no longer current once all documents are released. Thus even the First World War is still on the radar as being partially current, and not yet fully historical. My belief is that there are probably no Boer War documents waiting to be declassified, but substantial quantities of documents relating to the First and Second World Wars, and the Korean War, remaining in British and U.S. archives waiting for their declassification clock to wind down.

Adding to the two existing answers, another important aspect is hindsight.

As much as some events are clearly historical in nature, it's very difficult to foresee or analyze how the dots connect with follow-up events (related or not), even with hindsight. On occasion one only grasps the full consequences of current events 50 years later or more.

Sometimes it's even centuries. Take e.g. the incessant fighting over Palestine, the Kurdistan-related conflicts, the recent Balkan wars, or the ongoing Ukrainian civil war. They're but recent examples of conflicts whose origins you can trace to "unfinished business" from a century ago or longer.

There are many potential hotspots today, in fact, that could - who knows - eventually erupt in the future. To name but a few, take Hungarian irredentism, Armenian irredentism, the China-Russia border question, the South China Sea dispute, or even - as crazy as it may sound - the US-Mexico border question.

Sticking to the latter example, and getting back to your question: are historians able to write about the US conquest of Mexico 170 years later? Of course. Can they do so without any politics being involved in it? Probably not. Only the future can tell if an event truly is "finished business" or not. Politics is an integral part of the conversation until it is, at every step in between, and even after it is (given that history is written by the victors).

Anything that has already happened is a fit subject for history. Some topics can be studied from documents; others from oral records; and others from archaeological remains, such as pre-Colombian Indian civilizations, or pre-Roman Celtic and German peoples. So much for my opinion.

Philosophy of History starts with "History is the study of the past in all its forms." They go on to quote Thucydides:

“To hear this history told, insofar as it lacks all that is fabulous, shall perhaps not be entirely pleasing. But whoever desires to investigate the truth of things done, and which according to the character of mankind may be done again, or at least approximately, will discover enough to make it worthwhile” (Thucydides, The Peloponnesian War I, 22).

The work of a historian is not limited to finding information, but should include organization and analysis. Herodotus may be the Father of History, and his great work has much valuable information, but Thucydides is the first historian.

On your subsidiary question, about the influence of political considerations in the works of historians - a photographical study of the works of historians will always find some form of bias. It is part of the human condition.

For a more recent example, consider History of United States Naval Operations in World War II, by the Nobel Prize winning historian, Samuel Eliot Morrison, who began work in May of 1942, shortly after the start of the war! Following in the footsteps of Thucydides.