This question can have different answers depending on your own take with a romantic relationship. Some people can have a romantic relationship as early as fifteen or even younger than that specific age, while some chose not to be in a romantic relationship until the age of eighteen.
Some states in the United States specifically provide the proper age for marriage, but it does not necessarily mean that a person cannot enter into a romantic relationship before reaching that age of consent. However, for the purpose of answering this question, I believe that a person who is younger than 15 is considered as “too young to be in a romantic relationship.” This is because this is the stage wherein people are still not firmly grounded and are more prone to making foolish decisions and actions.