Simply dating a person over age 18 is not illegal. It can become illegal for a person who is 18 once sex is involved. But the situation you describe is not illegal.

Can a 17 year old boy date a 15-year-old girl?

People of any age can date each other. If either of them are under age their parents have to agree to the dating. It is never illegal to date with parental approval. However, each state has their own rules about sex and intercourse so as long as there is

Is an 18 year old dating a 15-year-old weird?

In a Nutshell: Sex between an 18 year-old and a 15 year-old is illegal in California, meaning any touching of “private parts” of either, even with consent of both parties (and perhaps even the parents of each teenager) is illegal in California because the age of consent is 18.