I think Judith actually went into labor during O-levels rather than A's, if memory serves, making her even younger--sixteenish? For some reason the detail of "during an Art O-level" sounds more middle-class to me, I suppose because of its bourgeois uselessness.
Felix I tend to imagine as another havoc-causing teenager at the time, perhaps on some sort of a study-abroad program from his Swiss gymnasium? although the eventual description of him does make him sound a bit older. (If the genders had been reversed, one could imagine him as another version of Claudie.) You're absolutely right that he comes off as a well-to-do gent. (One wonders if he never came up with a stepmother for Edward?)