LXX Num 1:45

καὶ ἐγένετο πᾶσα ἡ ἐπίσκεψις υἱῶν Ἰσραὴλ ἐν δυνάμει αὐτῶν ἀπὸ εἰκοσαετοῦς καὶ ἐπάνω, πᾶς ὁ ἐκπορευόμενος παρατάξασθαι ἐν Ἰσραήλ,

So all the Israelites twenty years of age or older who could serve in Israel’s army were counted according to their families.