You seem to be interested in the oldest Italian text readable by a native speaker. Unfortunately the problem is that there just aren't many old Italian texts, especially from Tuscany. Here I'll list the main ones I could find (my source is Migliorini's Storia della lingua italiana), limiting to texts coming from Tuscany and neighboring areas, together with some comments about how easy or hard I found them to read.
The first document arguably in vulgar Tuscan I could find is the postilla Amiatina. It is just a short note at the end of a donation certificate in favor of a monastery on mount Amiata (dated 1087) that reads
Ista cartula est de caput coctu
ille adiuvet de illu rebottu
qui mal consiliu li mise in corpu
To be honest this looks a lot more like vulgarized Latin than Vulgar. Migliorini conjectures it is an attempt to "latinize" the following words
Esta carta è de Capucottu
e ll'aiuti dellu' rebottu
che mal consigliu i mise in corpu
Its interpretations is in any case rather obscure, because of the uncertain meaning of the word rebottu. To be honest I'm in doubt that one should even count this, and I'm including it only for completeness.
Next comes the Ritmo Laurenziano. This is a longer text, you can find it here. It dates to the mid twelwth century and it's not easy to read (mainly for the lacunae). It begins as
Salva lo vescovo senato, lo mellior c'umque sia na[to],
[. . .] ora fue sagrato tutt'allumma 'l cericato.
Né Fisolaco né Cato non fue sì ringratïato,
e 'l pap' ha·ll [. . .-ato] per suo drudo plu privato.
There are a few words that are definitely archaic and might require even a moderately educated speaker to open a dictionary (umque "never", drudo "friend"), and the spelling is definitely non-standard (cericato?). I would say that this is borderline readable. I suspect that without the lacunae and with a little more context it would be easier to guess the meaning of the unfamiliar words. It also uses several words in very "Latin-like" form, presumably because it was intended for the court of the bishop, where Latin would have been very common.
The next document is the Ritmo lucchese. This is a short poem describing a fight inserted in the middle of a history (in Latin). It is dated around 1213. It is too long to transcribe here (the complete text can be found here), but let me copy a portion here:
Stiano a mente, ben lo dico:
che a Lucca sempre sia schifato
e a Lucca sempre sia odiato;
aver di Lucca non i sia dato;
tolto i sia quel che a pilliato,
ka di Lucca l'à 'nvolato:
tutto fu dello sacrato!
In my opinion this text, while definitely archaic, is already perfectly comprehensible to an educated Italian speaker.
After this the next available documents in Tuscan are the literary works of the Dolce stil novo school. These are slightly more archaic than Dante's Commedia, but not significantly so and I would argue are already perfectly readable by any educated Italian speaker. Arguably we should lump with them the Tuscanized transcriptions of the poetry of the Sicilian school, which are either contemporary or shortly before them.