Benjamin is born under highly unusual circumstances in 1860s Baltimore, entering the world with the physical appearance and demeanor of an elderly man. He ages in reverse, moving backward through the physical stages of life while historical time moves forward. His condition creates constant friction with societal expectations, forcing him to adapt his behavior to match his changing physical presentation. He seeks acceptance and a normal life despite his baffling physical trajectory.

Roger is a wealthy hardware business owner in antebellum Baltimore who places immense value on social standing and reputation. He responds to his son's unusual birth with panic and anger, prioritizing the opinions of high society over the reality of the situation. He insists on treating his elderly-looking newborn as a typical infant, forcing conventional childhood activities upon him.

Hildegarde is a beautiful young woman in Baltimore society who possesses a strong preference for older men. She finds middle age to be the ideal period of a man's life, which draws her to Benjamin when he appears to be fifty years old. She stubbornly refuses to believe the unusual truth about his condition, preferring her own assumptions about his age.

Roscoe is Benjamin's son, born during the middle years of Benjamin's life when he appears physically typical for a new father. Like his own grandfather, Roscoe is deeply concerned with societal approval and business efficiency. He grows increasingly frustrated and humiliated by his father's physical regression, viewing it as a deliberate affront to his own success and masculinity.

Benjamin's grandfather is an elderly man living with the Button family during Benjamin's early years. While Benjamin's parents are mortified by the child's elderly appearance, the grandfather provides Benjamin with a rare source of companionship. They function as peers, spending their time together like old friends.

Benjamin's grandson is Roscoe's first child. Initially kept at a distance from Benjamin, the boy eventually becomes his primary playmate when Benjamin regresses to an apparent age of five. They attend kindergarten together, sharing activities that fit both of their physical ages.

Mrs. Button is part of the moneyed Southern gentry in Baltimore. She gives birth to Benjamin in 1860, experiencing the immediate confusion and distress surrounding his unusual arrival. She shares her husband's concern for their enviable social and financial position.

Nana is a nurse employed by Roscoe's family. She is responsible for looking after young children, teaching them basic words and guiding them through toddler games. Her duties expand to include the physically regressed Benjamin.