We know Chaucer (e.g. Canterbury Tales) to be difficult, and it looks like this is the earliest that we can expect to fully comprehend a complete work (with difficulty).
I’m reproducing this Quora post below because it’s so interesting.
What is the oldest text a modern English speaker can read and understand?
There isn’t a bright line in history that says “before this date, you can read it and after this date you cannot”. Rather, it just gets easier and harder, with varying degrees of stuff you just can’t figure out without a key.
Old English is completely unreadable to a modern English speaker. So before the Norman Conquest, you have no chance at all. After the Conquest, grammar, word order, and vocabulary began to be increasingly influenced by Norman French, and features that had been present only in certain English dialects became more standard. This is a passage from the Peterborough Chronicle, dated from 1135. This represents the very end of the Old English period, which most scholars put at 1150. The things that look like funny Ps and Ds have a TH sound.
On þis gære for se king Henri ouer sæ æt te Lammase. And Ðat oþer dei þa he lai an slep in scip, þa þestrede þe dæi ouer al landes and uuard þe sunne suilc als it uuare thre niht ald mone, an sterres abuten him at middæi. Wurþen men suiðe ofuundred and ofdred, and sæden ðat micel þing sculde cumen herefter: sua dide, for þat ilc gær warth þe king ded ðat oþer dæi efter Sancte Andreas massedæi on Normandi.
Good luck reading that, although you can get a decent number of individual words. Here’s a translation.
In this year, the king Henry went over sea at Lammas. And the second day when he lay asleep on (his) ship, then the day darkened over all lands and the sun became such as if it were a three-nights old moon, and stars about it at midday. Men were greatly astonished and afraid, and said that a great matter ought to follow hereafter: so it did, for that same year the king died the second day after Saint Andrew’s mass-day in Normandy.
Middle English was a period of massive transition in vocabulary, grammar, spelling, and pronunciation. The passage above was notable for spelling king as it does. Just a few years before, it would have been cyning. Just a few years after, all of the funny looking letters would be largely gone.
Very early Middle English is almost as unreadable as Old English. This is a passage from the Ancrene Wisse, a guide for women in religious life, written in the early 1200s. Where you see a 3, that represents the old letter yogh, with roughly a consonant Y sound:
The an riwleth the heorte, ant maketh efne ant smethe withute cnost ant dolc of woh inwit ant of wrei3ende the segge, ‘Her thu sunegest’, other ‘This nis nawt ibet 3et as wel as hit ahte.’ Theos riwle is eauer inwith ant rihteth the heorte.
You can fight your way through that and pick out a few words. I doubt you got the overall meaning. Here is the passage translated:
One of them rules the heart, and makes it even and smooth without the bumps and hollows of a crooked and accusing conscience which says, “You are sinning here”, or “This is not yet atoned for as well as it ought to be.” This rule is always internal and directs the heart.
But by the time of Chaucer 150 or so years later, you had this:
When that Aprille with his shoures soote
The droghte of Marche hath perced to the roote,
And bathed every veyne in swich licour,
Of which vertu engendred is the flour;
Whan Zephirus eek with his swete breeth
Inspired hath in every holt and heeth
The tendre croppes, and the yonge sonne
Hath in the Ram his halfe cours y-ronne,
And smale fowles maken melodye,It is hardly crystal clear, but the average person can get it. Even then, you will need some help (or context) to know what a heeth is.
There is an additional complication that English was very heavily dialectical at this point. You’d have an easier time reading material from the East Midlands (the dialect that ultimately gave rise to Modern English) than you would anything written in the far north or south.
However, that doesn’t really answer your question. A decent answer is that by the mid-late 1300s, you can fight your way through a Middle English text and pick up the meaning, even if you drop a few words here and there.
Pictured are the levels of linguistic structure.
What? How? Wonder. 