Usage: 2 forms in 2 verses
Form Translated As     In Verse(s)
πρεσβείου of a senior LXX.Ps.71.18
πρέσβεια their seniority LXX.Gen.43.33

Liddell-Scott-Jones

πρεσβεῖον (presbeion) senior G:N-N

πρεσβεῖον-εῖον,

Ionic dialect and Epic dialect πρεσβεῖον-ϊον, τό, gift of honour, πρώτῳ τοι μετ᾽ ἐμὲ πρεσβεῖονϊον ἐν χερὶ θήσω [πρεσβεῖονήϊα τέχνης “Anthologia Graeca” 9.656.6. ">Refs 8th c.BC+]

__2 privilege of age, [Refs 4th c.BC+]: hence, generally, privilege: plural, prerogatives, πρεσβεῖονεῖα διδόναι τινί, with infinitive, give him as a privilege, to.., [πρεσβεῖονείων ἐπιλαμβάνειν 4th c.BC: Aristoteles Philosophus “Ethica Eudemia” 1242a6: with \_genitive\_, γῆς πρεσβεῖονεῖα">Refs 5th c.BC+] chief share or sovereignty of the land, [Refs 5th c.BC+]
__3 right of the eldest, his share of the inheritance, πρεσβεῖονεῖα λαβεῖν [Refs 4th c.BC+]; κατὰ τὰ π., ={κατὰ πρεσβεῖονείαν}, [LXX+1st c.BC+]
__II old age itself, [LXX]

LSJ Scripture Refs: LXX.Gen.43.33LXX.Ps.70.18*