In Italy we all study Dante, Petrarca and Boccaccio in school, which are 1300, and it's quite easy to understand them beyond some unusual words. 1200 poetry is easy enough too.

There's not much literature older than that, cause people preferred to write in Latin, the oldest bit in "volgare" is the Indovinello Veronese[0] which is from the 8th or 9th century and at that point it's almost latin spelling-wise, it's understandable if you're well educated but wouldn't be understandable by everyone.

[0] https://en.wikipedia.org/wiki/Veronese_Riddle

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.

People read Shahname[1] regularly in Iran, and it was written at around 1000 CE, but there isn't much before 900 CE that is comprehensible to a modern day Persian speaker.

[1] https://en.wikipedia.org/wiki/Shahnameh

The Shahnameh is a long epic poem written by the Persian poet Ferdowsi between c. 977 and 1010 CE and is the national epic of Greater Iran. Shahnameh is one of the world's longest epic poems, and the longest epic poem created by a single author.

If you're interested you can read up on language change (and glottochronology, although that's a bit controversial now), and the Swadesh lists.

In general, language changes around at the same rate all over history and geography, barring some things (migration, liturgical languages)

This is, roughly, a measure of how old your civilization is.

For square Hebrew (Assyrian) you can go back for about 2000 years. So for example Dead Sea scrolls are fairly readable. But old classical Hebrew impossible.