The son of Polo Baggins, Posco married Gilly Brownlock, and together they had three children. The first was named after Posco's grandfather Ponto, while the second and third were named Porto and Peony. Posco was born in the Shire-year 1302, and though he may well have lived into his old age, he apparently didn't get old enough to celebrate Bilbo's Farewell Party ninety-nine years later. We know that his wife Gilly was present at that Party, but Posco was not.
As a Baggins, Posco was related to both Bilbo and Frodo, but only very distantly. His closest ancestor in common with Bilbo was his great-grandfather Balbo Baggins, who was also Bilbo's great-grandfather through a different line of descent.