Is it against the law for a 15-year-old and an 18-year-old to be in a relationship?

Yes. * It depends upon the nature of the relationship. There are not laws that address the ages of persons who date. There are age of consent laws that pertain to sexual relationships of minors and adults regardless if the sex is consensual. The minimum AOC in any state for a minor engaging in consensual sex is 16, in many states it is higher.