15th? Ye-es... maybe, although they would struggle greatly with it and need the motivation to do it, which would be key. Certainly not with perfect, easy comprehension--they'd need a guide or dictionary or teacher to answer questions, and many hours of work.

If they're able to speak French as well, they'll have an easier time, but not by too much.

If the text is modernized it's much easier. If it looks like this, well:

http://www.maloryproject.com/image_viewer.php?gallery=Winchester&image_id=11&pos=11

This is harder.

(I'm an average English speaker who happens to be studying this in graduate school. I would class my English skills as "reasonable but not excellent".)