Obie jesteśmy dorosłe.

oba (male non pers and neuter) (nom, voc, acc) = both (num, plural only)
obaj (male personal) (nom, voc)
obu (all genders) (gen, dat, loc)
oboma (male personal, male n.p. and neuter)
obie (female, nom, acc, voc)
obiema (female instr)
oboje (coll num) ‑jga = both (Collective numbers are used before nouns denoting people of mixed sex, personal and animate neuter nouns (of which there are few), and non-masculine personal nouns which are plural only. They are used with nouns in the genitive.E.g. oboje studentów (both students, one male, one female)