It's from an an essay written as a hypothetical commencement speech by columnist Mary Schmich, originally published in June 1997 in the Chicago Tribune.

Then released in several versions as a spoken word song by the Australian director Baz Luhrmann.

https://en.wikipedia.org/wiki/Wear_Sunscreen

It reached peak parody when further mangled and released by John Safran as Not The Sunscreen Song https://www.youtube.com/watch?v=8YwqFz14xY4