Born sometime in the 19th century, Jack is the son of archaeologist Lorenzo Cabot. In 1894, his father led an expedition to Arabia in search for the fabled Ubar civilization. After Lorenzo returned to Boston from the Middle East with an alien artifact, he was found to have otherworldly powers. For the safety of him and his family, Jack imprisoned the archaeologist and his artifact at Parsons State Insane Asylum in 1898.
Jack devoted his life to studying the artifact. His research into the mysterious object subsequently resulted in the development a serum derived from his father's blood that defies aging, in other terms halting, or at least, slowing down the process. In turn, Jack's special serums grants him and his family the ability to live for more than four hundred years.
If the player character sided with him during the quest The Secret of Cabot House, Jack will provide caps and, after an in-game week has passed, Lorenzo's Artifact. Should the player visit Parsons after this point, Jack may appear older.
The player character might have a random wasteland encounter in which Jack and Edward appear either in the player character's settlements or on the road. Jack offers to purchase any fiber optics the player character might have, which are necessary for a research project he will not discuss. This seems to happen right after the player gains the perk Scrapper 2 and scraps guns to gain fiber optics.
Siding with Jack will cause him to permanently stay in the Parsons Asylum basement unless the player character decides to free Lorenzo before killing him.
Some in-game time later, Jack's appearance may change drastically (like his sister's) and he will appear old. He may also comment on his mother's death, mentioning that she could not survive long without the serum.
At one point, Jack Cabot will guess the Sole Survivor's age, he himself being 400 years old, he will estimate the Sole Survivor to be at least 200 years old, guessing they were born mid-21st century. The Survivor can say he's correct, deny, or evade the question, or inquire as to how he came to this conclusion, to which he replies their demeanor, appearance, health, and manners are a dead give away. This puts the Sole Survivor's birth date reasonably anywhere from the early 2040's to mid 2050's, given their accomplishments in civilian life prior to the Great War.
When initiating his quest line, his dialogue to the player character includes the line, "but that's a story for another time," and the phrase "hidden histories." This is a nod from the developers to the Shoddycast YouTube channel, which prominently features Bethesda games among other assorted franchises.
If the player character chooses the "I'm Impressed" dialogue option when the player accepts the Artifact gun, he will reference GLaDOS from the Portal series "Still Alive" end credit song with the line "You made a neat gun!"
If the player character immediately leaves after speaking with Deegan in the quest Special Delivery, Cabot may start following them across the Commonwealth. He will return to Cabot House once spoken to.
After receiving the artifact gun from Jack, if the player character chooses to ask what Jack will do now, Jack will say he will travel southwest, for his father had always believed there was an ancient city below the Mojave.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
The Cabot family is based on one of "Boston's First Families" the actual Cabot family (John and Joseph Cabot). They were a very well-off family of shippers and traders, who moved to Boston in the 1700s.
Jack Cabot's appearance and mannerisms are based off real-life rocket scientist Jack Parsons, known for his experimental work with rocket propulsion in the 1940's and for being a leader of the OTO, a religious group which practices magic and rituals.
End of information based on unverified behind the scenes information.