It is very hard to know know when more than 50 % of Humans over the age of 16 knew that the Earth is roughly spherical.

The two regions with the largest populations are China and the Indian subcontinent.  It is quite possible that at various times they had a total of two thirds of the world's population. 

It is said that intellectual elites in India learned that the world is spherical in the Gupta Era and so by about the time of the fall of the Gupta Empire in AD 543, and that intellectual elites in China learned that the world is spherical in the 17th century (1601-1700).

But how rapidly did such fascinating knowledge with little use in everyday life spread from intellectual elites to more and more common people until the majority of adults knew that?  How many years, decades, centuries, or millennia would it take? 

European sailors began circumnavigating the Earth in the 16th century and news of those voyages was big news in Europe.  And of course many of those voyages reached ports in highly populated regions of Asia, where the voyagers may have told the locals that the world is spherical and they were sailing around it.  So how much did that news spread in those regions? 

https://en.wikipedia.org/wiki/List_of_circumnavigations[1]

And eventually mass education programs were introduced in India and China beginning in the 19th century and much more so in the 20th century and the majority of the people there received education by the time they were adults.  Including some basic science, I presume.

And the space age began in 1957 with the first satellites, and presumably everyone  who explained to someone what a satellite was had to explain that the Earth is spherical if the other person didn't already know.  And a few years later famous pictures of the spherical Earth began to be taken from space, often with recognizable parts of continents as seen on maps and globes.  

So by the end of the 20th century 19 or 20 years ago the vast majority of adults and schoolchildren in the world knew that Earth more or less shaped like a sphere.

But it seems uncertain to me when the percentage of Humans above the age of 16 (as asked in the question) that knew that the Earth was roughly spherical first reached and exceeded 50 percent.  Was that in the 20th century or in the 19th century or possibly even earlier?


  [1]: https://en.wikipedia.org/wiki/List_of_circumnavigations