I have found at least one website that claims the phrase was a reference to
Simon's girl friend because she had found what she thought were some gray
hairs.  It was meant as a joke.  Nothing like joking when you are trying to
write a serious song that, in 1970, was off the charts.  Nobody that did
dope back in those days that I knew remotely believes such an
interpretation.  I think the boys wanted to cover their butts since the song
became so popular.  Frankly, I personally could care less what they wanted
it to mean.

Phil.