All of my great-grandparents had died before I was born. So the middle person will have to be one of my grandparents. Here are the earliest born ancestors that each of my grandparents may have known.
Joseph Andrew Huigens (1891-1957) had to have known his grandmother, Anna Helena (Schmitt) Fitzler (1833-1904). They both lived near Creighton, NE according to the 1900 US Census. He would have been 13 when she died. It's possible he also met his other grandmother, Katherine (Toemmes) Huigens Wend (1827-1899). She died in 1899, when he was eight. In 1895 she was living in Iowa with her daughter's family. She died four years later in a mental hospital in Illinois.
Mary Julia (Kain) Huigens (1894-1979) only had one grandparent who did not die before she was born, her maternal grandfather, John Kennedy (1826-1913). She would have been 19 when he died. I have no idea if she ever made the trip to Clinton, IA to meet him, if he ever came out west to Nebraska or if they met somewhere in the middle in Iowa for a family gathering at her aunt's.
Emery Julius Huigens (1896-1984) probably knew all of his grandparents. He was 14 when the first of them died. They all lived near each other in south-eastern South Dakota. His paternal grandfather, James Robert Ogden (1836-1910), was the oldest and the first to die. Emery would have been 14 then. It's also possible but unlikely that he met his great-grandfather, John Jacob Adlebusch (1817-1901). He lived in Sheboygan Co., WI and was almost 80 when my grandfather was born. I doubt that my grandfather was taken to Wisconsin or that my great-great-great-grandfather made the trip to SD at his age.
Esther Louise (Johnson) Ogden (1902-1984) probably knew all three of her grandparents who were alive when she was born. The oldest of them was Harriet Alice (Howard) Strayer (1829-1914). They all lived in the Quad City area in Iowa and Illinois where my grandmother was born.
So I'm pretty sure the farthest back I can go is Esther Louise Johnson to her maternal grandmother, Harriet Alice Howard, born in 1829. There are two other definite possibilities in 1826 and 1827 with a very, very slim chance of 1817.
Thanks, Randy, for another great topic.