In my History lessons (and I'm sure in other people's) we studied modules e.g. America 1919-1929 or Peacemaking & The League of Nations: 1918-1939 (both of which were actual modules I've actually studied).

In some ways it can be a fun way to get a snapshot of an aspect of History however other times (which is the majority of the time for me) can be extremely annoying as it reinforces the idea that the past isn't a continuous thing that event X started in 1933 which ended with Event Z in 1945. To me this way of viewing History (however efficient it may be for drawing a line under what one needs to study for exams) isn't fun. It makes History seem boring (and probably the reason why History pretty much bored me until I found this subreddit)

The way subjects are taught in the UK (not sure about the rest of the world) is that you're really only learning about (for example) the RAD ("Reichsarbeitsdienst"), KDF ("Kraft durch Freude") or the Lebensborn program to answer the question: Did the German people benefit from Nazi rule in the years 1933-1939? (which was an actual question in one of my History exams). You're not learning that information because it's simply history & you may find it enjoyable, you're learning it to pass an exam which won't even matter 5 years from now.

how far back do we have to go to pick up the "starting point" for the modern world? Or can we never escape the whole of human history and its impact on us today?

I can see why exam boards choose a starting point, because it makes History easier to digest & a means of simply getting the facts on a need to know basis, e.g. Someone who's going to answer the question on the benefits of Nazi rule needs to know about the KDF but they don't need to know Hitler got the idea from Mussolini's OND ("Opera Nazionale Dopolavoro").

Every action has a reaction. In the UK (especially) when a student gets to GCSE level (ages 14-16) you start having "starting points" because it makes the specific course module more concise meaning you can learn about several different periods of History (as well as touching on briefly how one event affects another e.g. the effect the Great depression in America had on the rise of Hitler) in a short space of time.

By the time a student reaches A-Levels (ages 16-18) you focus more in depth on a smaller amount of time periods (e.g. "Democracy & Nazism in Germany; 1918-1945", "The British Empire: 1847-1967").

For an A-Level module like "The British Empire: 1847-1967" you'd briefly touch on the effects the Battle of Waterloo had on the world this would include things like what happened in 1815 & how that effected the world in the next 100 years (give or take). Once you been brought up to 1914 you'd start to realise the effects the "past" had & how that effects what's to come.

You'd be introduced to the ideas of the "Concert of Europe" & the "Balance of Power", you'd start to see that even though exam boards try to get you to believe there are start & end dates, there really isn't. You'd see the bigger picture; that even though event X started in X-date & ended with Event Z in X-date it still has effects on how we view our world today.

John Aldred states in his British Imperial & Foreign Policy: 1846-1980:

The foundations of British foreign policy towards Europe had been laid in 1815. The European great powers whom Britain was one, had established the "Concert of Europe". Commitment to this was to be of primary importance to Britain for at least the next 50 years.

The concert was purely a European arrangement [after being devised in the "Congress of Vienna"]. But Britain's interests expanded well beyond Europe. Nevertheless throughout the nineteenth century there was a direct link between order in Europe & the protection & expansion of the British empire.

A strategically and diplomatically stable Europe not only enabled Britain to retain it's status as a regional and continental power, but also as an imperial power. This relationship between European peace and British imperial power was a central factor that drove most of Britain's foreign & imperial policies throughout most of the 19th century. There was a degree of consistency in the principles on which the policies were based but the methods employed to ensure Britain's vital national interests were guaranteed, changed over time.

Aldred goes on to say this:

From 1846 to 1902, one of Britain's primary objectives was to maintain the "balance of power" [...] The "balance of power" was a theory that peace and stability are most likely to be maintained by ensuring that no single or group of great powers should be able to become strategically, territorially or diplomatically capable of dominating the other powers in a region.

Norman Lowe writes this in his Mastering Modern British History:

Britain in 1815 was a victorious nation apparently at the height of it's power & prestige. But there was no tranquility after the turmoil of the previous 22 years; the period following the Battle of Waterloo turned out to be crammed with important changes & developments. [...] These changes brought with them innumerable problems; poverty, discontent & violence.

So as you can see on the contrary to perhaps popular belief, the "birth" of the modern world (and what defines it) is said to begin in 1815 with the end of Waterloo & the decisions of the "congress of Vienna" not in 1914 & WWI.

As one gets higher in education (undergraduate, postgraduate etc) the way we view how things start & end (and why they start & end in they way they do) doesn't become as clear cut as we are originally led to believe, it becomes a mind field of various different interpretations where nothing is every truly right or wrong. Where nothing ever really begins or ends.

TL;DR: In lower levels of History Education (especially in the UK), we are led to think things "start" & "end" and once it's over then that's that but as we climb higher & higher in education we start to realise nothing is ever as clear cut as what we once thought, was simple becomes a mind field of different interpretations & opinions.

Welcome to the world of Academia.

Sources & Further Reading:

1. Heinemann Advanced History: British Imperial & Foreign Policy 1846-1980 ~ John Aldred

2. Mastering Modern British History (2009 edition) ~ Norman Lowe