There are two known Amano drawings of Sabin with a beard.
500full.PNGSabin_II.PNG

They say a picture is worth a thousand words...how did this legendary beard come to pass? When did he grow this beard? When did he shave it? Why?

It doesn't appear in the game itself, so if he is supposed to have grown this beard, it would seem to have happened off-screen.

*Paid for by the American Society for the Study of Beard-Related Phenomena, aka the Chuck Norris Foundation*