A serendipitous and amazing occurrence will take place for the first and only time in history on 31 December 2017.
As one intuitive Twitter user known as Will, or @WMcHBg, pointed out, on New Year's Eve this year...
...all living adults will have been born in the 20th Century, while all living children will have been born in the 21st Century. This is the only day in history where this will be the case.
However, the same thing has been tweeted prior by other accounts:
Let's break this down.
Basically what Will is saying is that everyone born on or before 31 December 1999 will have turned 18 years of age by 31 December 2017, thus meeting the minimum requirement for being considered an "adult".
Anyone born after that date - 1 January 2000 and onwards - will still be seen as a child in the eyes of the law for that day.
And if we consider that the oldest living person on the planet is 117-year-old Nabi Tajima of Japan, and that she was born on 4 August 1900, it means that no one born prior to the 20th century is still alive.
So that does indeed mean that on 31 December 2017 - for one day only - every living adult on the planet will have been born in the 20th century, while everyone born thereafter - in the 21st century - will still technically be a child.