How were people able to graduate with a PhD at a such young age in the past? When I read up the bibliography of the mathematicians and scientists in the early 19th to 20th century, I am always shocked at how they were able to graduate university with PhD(s) at such a young age.

For example, John Von Neumann graduated from two universities simultaneously at the age of 22 with two PhD degrees and critical papers published.

John Forbes Nash graduated with a PhD at 22 on game theory. Enrico Fermi finished school at 21 with thesis on X-ray diffraction. Kurt Gödel finished school at 23 with dissertation on predicate calculus. Abel finished at 20 with thesis on quintic equations. Galois never graduated but created a field of his own before dying at the young age of 20 and the list goes on and on...

I am not knowledgeable of the trend in the social sciences but I suspects much of the same. 


Can someone tell me if there has been a major shift in how university
education is conducted such that university across the world takes
longer to get through than it used to be  
or is it truly because
these people are unusually talented and are able to get through
university purely based on their almost unnatural abilities?


Most importantly (and relevant), why is there such a decline in these young PhDs (I've never heard of a professor at my university who graduated at 21) and is it still possible for people to graduate at such a young age nowadays?
There are several issues:


Your examples are somewhat biased, since you've selected researchers who were not only brilliant but also prodigies.  By contrast, Riemann finished his Ph.D. dissertation at the somewhat older age of 25, and Weierstrass didn't start his research career until he was 39 or so.  Brilliant researchers are more likely to get an early start than average researchers are, but the correlation is far from perfect.  Focusing on prodigies gives a misleading picture.
There are plenty of prodigies today (Terry Tao, Noam Elkies, Akshay Venkatesh, Erik Demaine, etc.).  If they occupy a smaller fraction of academia, it's probably because academia has grown enormously even relative to the population.  The faculty in the top departments today are stronger on average than a hundred years ago, but there are many more departments nowadays, so the average professor is overall probably not as talented.
The amount that's known is steadily increasing, so getting to the 19th century research frontier takes quite a bit less time than getting to the 21st century research frontier.  This is not as relevant in very new fields, but finishing a Ph.D. in algebraic geometry in two years is a more impressive feat today than it was a hundred years ago.  This increase in knowledge doesn't translate perfectly into an increase in time to degree: people also compensate by specializing more and then broadening later in their career.  However, the obstacles to a quick Ph.D. are certainly increasing over time.