the essays that are handed to me are written by "mature students," typically people in their mid-forties and upward.

Just because people are "mature students" shouldn't mean that they learned to spell any better than us young folk. Some people, both young and old, just don't care if things are spelled quite correctly - or punctuated correctly. (I DO care, by the way.) Otherwise "mature" shop owners wouldn't write things like:

potato's - .99¢/ lb

(which would imply 100 pounds to the dollar)