Manhattanhenge must have been a phenomenon for as long as Manhattan has had lots of tall buildings - so presumably for at least a hundred years by now. And yet it was not called "Manhattanhenge" until about 8 years ago, if Google Trends are to be believed.
Today and July 12 are "full sun on the grid" days of Manhattanhenge.
Apparently Stonehenge itself was not paid much attention to until about 90 years ago.
In this video, Neil DeGrasse Tyson seems to be taking credit for coining the term.