Imo Warcury or Ju Peter could be one of the og 20.
The other Gorosei are their descendants from different eras.
Tbh i dont think so. Saturn died immediately once his powers were taken from him, which implies he didnt receive any immortality in the way Imu has it (staying young), probably due to whoever had Law's fruit in his time.
If you look at it, the Gorosei all are older men who get promoted through abyss contracts. We can see Garling aged and presumably now stops aging once he became a Gorosei. Sommers, him and Shamrock all showing that they still age even as Knights.
The only actual weird one is Gunko, because she hasnt aged in 50 years