oldness vs seniority

oldness and seniority both are nouns.

Word	Noun	Adjective	Verb	Adverb
oldness	Yes	No	No	No
seniority	Yes	No	No	No

As nouns, seniority is a hyponym of oldness; that is, seniority is a word with a more specific, narrower meaning than oldness:

Other hyponyms of oldness include agedness, senescence, senility, longevity, staleness.

oldness (noun)	seniority (noun)
the quality of being old; the opposite of newness	the property of being long-lived
the opposite of youngness	higher rank than that of others especially by reason of longer service