Why did so many of my classmates get prematurely gray? I had no idea until I saw them on facebook, and they are barely 60.

Bell shaped curve. The development of grey hair follows a statistical curve with a new in their 30's gettting grey, more in the 40's and so on. By the seventies most have turned grey with a few rare exceptions. Greyness is common in the 60's and your friends are probably not prematurely grey.