The oldest clear associations come post E. Levi's Kabbalistic associations. By linking each letter of the Hebrew alphabet to the Major Arcana, astrological associations result.
The reason is that, for example, the 12th c. Sefer Yetzirah, the oldest of the three 'classical' Kabbalistic texts, links each of the single letters with a zodiacal sign, each of the seven doubles with a 'planet', and each of the three mother-letters with an element.
Depending on where one places Aleph (which is the first letter and also means 1), different correlations will follow. Waite, following the Golden Dawn, places Aleph on the Fool, and the implications this has.
On a different note, the Visconti-Sforza (15th c) deck's depiction of the Magician is virtually the same as a miniature from an astrological depiction of Mercury from the same era, and and made for the same family. As far as I am aware, this is the earliest possible connection.
Most designers have followed the Kabbalistic connections first promulgated by Levi, or modified by the Golden Dawn.