Peck has said that "a young character's climb to maturity" is the constant theme of the adolescent novel and that his special theme in all of his novels has been "you will never grow up until you begin to think and act independently of your peers." In Those Summer Girls I Never Met, main character Drew Wingate stands in contradiction to that comment. He has no peers other than his younger sister, Stephanie, who accompanies him on a cruise at the invitation of their grandmother. For a time, both are forced to live in an adult world.

The most significant characters in this novel are older people in their sixties and seventies. Most significant is the grandmother, Connie Carlson, a singer from the Big Band era. Because of an estrangement between Connie and the children's mother, Stephanie and Drew hardly know their grandmother. At first, both children resist Connie's efforts to thrust adulthood on them: She gives Drew a white dinner jacket and Stephanie her first pair of heels. As Connie tells Drew, "On a cruise, you're any age you can get away with." On board ship with their youthful grandmother, Drew and Stephanie live out their fantasies, but not without consequences and obligations. Peck, however, does not find this situation a cause for despair: In his view, freedom from is the domain of boredom, whereas freedom to is far more interesting.

The novel is also funny, largely through Drew's comic awareness of himself and his ability to stand aside and laugh at comic situations. The novel is also sad, however. Connie's cancer and the alcoholism of Drew's grandfather, Shep, are problems that can only be suffered through and to a degree understood.