I think it depends a lot on the history of the language. My native language is French, and since long ago various authorities try to normalize and "purify" the language. This is why the gap between spoken French and written French is so wide. Now my experience as an avid reader...

Books written in the 17th century or later are easy to read. Of course, the meaning of some words can change over time but that's a minor trouble. I believe Molière and Racine are still studied in school nowadays, but the first name that came to my mind was Cyrano de Bergerac (the writer, not the fictitious character).

Books from the 16th need practice, but I think anyone who tries hard will get used to the language. I enjoyed Rabelais's Pantagruel and Gargantua a lot, and I first read them by myself when I was in highschool (I knew a bit of Latin and Greek, which helped).

Before that, French was much more diverse; the famous split into "langue d'oc" and "langue d'oïl" (terms for "oui" — yes — at the time) is a simplification, because there were many dialects with blurry contours over space and time.

I've read several 11th-12th novels about the Round Table, but I was already experienced in Old French when I started, and I think most readers would struggle to make sense of it. It may depend on the dialect; I remember "Mort Artur" was easier than "Lancelot, le chevalier à la charette".

"La chanson de Roland" (11th century, Old French named anglo-normand) is one of my favorite books of all times. Reading it for the first time was a long process — I learned the declensions of Old French and a lot of vocabulary — but it was also fun, like deciphering some mystery. And the poesy is a marvel, epic, incredibly concise, surprising and deep.

Before that (9th-10th), Old French was even closer to Latin.