Do centuries end with 99 or 00? This is one of those number questions where the facts and the feelings clash. Of course it felt like the new century began at 2000 (when the big number switch 1999-2000 took place). But in fact (not feeling) there was no year zero and so each century lasts 100 years and actually finishes with 00.
Also, our country defines an adult as someone aged 18, but others don't. So the statement is only true based on our culture and values.
So, trying to anticipate all pedantry, the following statement is either going to be true on 31/12/17 or 31/12/18 or not.
And I am grateful to @bethbethbeth_ for telling us what her Dad told her. And to the ubiquitous @richardosman for making it more widely known. This is the thing:
'...on New Year's Eve, every single adult in the world will have been born in the 20th century and every child will have been born in the 21st century and that's the only day that's true for...'
Isn't that a beaut?