According to the 1767 census, Johann Georg Just was a Lutheran farmer from Darmstadt, Germany. That may not be his actual birth town, just the nearest major city. He was 66 years old in 1767, traveling with no children. His wife was 44 years old. Most likely any children he had stayed behind in Germany or settled in different Volga colonies. His wife Anna Elisabeth was probably his second wife.
Head of the Household Johann Georg Just, age 66, Lutheran farmer from Darmstadt
Wife Anna Elisabeth Just, age 44, probably second wife
He is not listed in the 1775 Grimm census.
Had he survived, he would have been 74 years old. Those early years were very difficult for the first colonists, and many died. He was probably one of them. If his wife was still living, she would have been 52 years old. Because of their ages, the couple probably had no children, and after her husband's death, she probably remarried.
Christening Place Evangelisch, Sprendlingen, Starkenburg, Hesse-Darmstadt
Father's Name Johann Georgy Justus
Pleve, Igor. Einwanderung in das Wolgagebiet 1764-1767, Band 2, Herausgegeben von Alfred Eisfeld under Mitarbeit von Sabine Eichwald, Published by the Nordost-Instsitut - 38085 Göttingen, 2005; page 74, family #4, Johann Georg Just family.
↑The 1775 and 1798 Census of the German Colony on the Volga, Lesnoy Karamysh, also known as Grimm; Published by the American Historical Society of Germans from Russia, Lincoln, Nebraska, USA; Published date: 1995; 1775 Grimm census.
"Deutschland Heiraten, 1558-1929," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:VCB3-XK1 : 11 February 2018), Johann Georg Justus in entry for Johann Philipp Justus and Elisabetha Catharina Stamm, 07 Aug 1737; citing Stadtpfarrei Evangelisch, Darmstadt, Starkenburg, Hesse-Darmstadt; FHL microfilm 1,190,505.