Where does “seniorem” come from?

seniorem (Latin) comes from Latin senex, from Proto-Indo-European sénos — old.

Ancestry of “seniorem”, step by step

Step	Language	Word	Meaning
1	Latin	senex	old, aged, elderly; old person, old man, old...
2	Proto-Indo-European	sénos	old

seniorem — Etymology, Origin & Word History | Etymology Explorer