Birthday Girl is often described as a steamy, age-gap romance with elements of forbidden and taboo love. Such romances deal with a large age difference between the protagonists. Typically, the male partner in a heteronormative love story is older by at least a decade. Age-gap romances also tend to have female leads who are quite young, typically between the ages of 18 and 28. In Birthday Girl, Jordan is 19 to Pike’s 38 years. An age-gap romance explores forbidden, boundary-pushing love. The love is depicted as so strong that it transcends the limits of age and social propriety. The age difference between the protagonists exists as an obstacle to be overcome in the path of true love. At the end of such romances, the unlikely pairing is happy and successful, showing that love conquers all.

In an age-gap romance, the younger protagonist seeks order and security from their older partner, while the older partner looks to the younger to provide a fresh chance and new perspective on life. In Birthday Girl, Jordan likes the fact that Pike runs his own business, has a beautiful house, and picks up after himself, all suggesting that he has life figured out.