Mental capacity and relationships

If an adult has, and will never progress beyond, the mental capacity of a teenager, is it predatory for an adult to pursue a romantic/sexual relationship with them? Or is it predatory for them to pursue a romantic /sexual relationship with a person close to their mental age since they are an adult?

For example:

Person A is 28 and has the mental capacity of a 14-15 year old, lives independently (with roommates) and works at a grocery store.

Person B is 28 and is a normal functioning adult and works as a foreman for a construction business.

Person C is 16, lives with parents, attends high school and also works at the grocery store with plans to attend community college.

Can person A be in an equal, non-predatory, romantic relationship with either B or C? Would you consider either relationship wrong? Or okay?