senex

Senex, senis is masculine, third declension, and works as both noun and adjective. It is not an i-stem: genitive plural senum, ablative sene. Note how short the stem is once the nominative is out of the way, and that the vowel changes: senex, senis, seni, senem, sene.

The comparative is senior, "older", which is the form that ran on into English senior, Spanish senor, French sieur and monsieur. In the medieval corpus senior is usually a title, an elder or a lord, and not a comparison at all, so reading it as "older" will produce nonsense in a charter. There is no regular superlative; Latin says maximus natu instead. Senatus, the council of elders, and senectus, old age, come from the same root.