Lee J. Cobb (born 1911), who played Frank Sinatra's father, was actually only four years older than Sinatra (born 1915). Tony Bill, who played Sinatra's younger brother, was 25 years younger than Sinatra. Molly Picon, who played Cobb's wife, was 13 years older than Cobb.
Maureen Stapleton turned down role of Frank Sinatra's mother. The part went to Molly Picon instead. According to one fan magazine, Stapleton (who'd just played Dick Van Dyke's mother in Bye Bye Birdie (1963), even though she was just a few months older than him) passed on the role because Sinatra was roughly eight years older than her and she didn't want to make a career playing mothers to her contemporaries.
Besides taking the lead role, Frank Sinatra also crooned the film's "Come Blow Your Horn" title song. It is the film's only musical number.