The characters in One Day in December try to make decisions informed by the context of their age and experience. Each time Jack or Laurie has the opportunity to pursue their love, they are at a different age and place in their lives. Laurie and Jack are on individual journeys trying to figure out how to navigate their many changing responsibilities, desires, and goals. Once they think they have one part of their life figured out, they quickly realize that there are other elements that require navigation. The coming-of-age theme is connected to the romance genre: By getting to know herself better, Laurie is able to figure out what she truly needs and wants from love. It takes years for her and Jack to get to a place of individual self-worth and joy, but when they do, it is because as they pass through their twenties, they become better at being honest with themselves.

The friendship between Laurie and Sarah starts to change not when Sarah begins dating Jack, but when Laurie and Sarah move out of their youthful, university-days apartment. In symbolically saying goodbye to that part of their lives, both young women must forge ahead with careers and love interests, not alone but certainly without the comforting safety net the other person provided for so long.