The legality and appropriateness of a relationship between an 18-year-old and a 14-year-old varies depending on the jurisdiction and specific circumstances of the relationship. In many places, there are laws that prohibit sexual contact between an adult and a minor under a certain age, typically 16 or 18.

In the United States, for example, the age of consent varies from state to state, but is generally 16 to 18 years old. If an 18-year-old engages in sexual activity with a 14-year-old in a state where the age of consent is 16 or 18, then they may be breaking the law and could be charged with statutory rape.
Even if sexual activity is not involved, there are still potential legal and social consequences of a relationship between an 18-year-old and a 14-year-old. In many cases, parents and other adults may view the relationship as inappropriate or concerning, and may take action to separate the couple or involve law enforcement.
Furthermore, there are often significant differences in maturity and life experience between a 14-year-old and an 18-year-old, which can make it difficult for the two individuals to have a healthy and balanced relationship. While there are exceptions to this, it is generally true that individuals at different stages of development may have trouble relating to one another in a romantic or sexual relationship.
It is also important to consider the power dynamics at play in a relationship between an adult and a minor. Even if the 18-year-old and 14-year-old are both willing participants, the older individual may have more power and influence in the relationship, which can make it difficult for the younger individual to assert themselves or make informed decisions.
In summary, the question of whether an 18-year-old can date a 14-year-old is complex and depends on a variety of factors, including the legal age of consent in the jurisdiction, the specific circumstances of the relationship, and the social and emotional maturity of the individuals involved. While there may be situations in which such a relationship is appropriate and healthy, it is generally advisable for individuals at different stages of development to pursue relationships with others who are closer in age and life experience.