Is it legal to date 15 and 17?

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 dating someone older than 18 illegal?

Simply dating a person over age 18 is not illegal. It can become illegal for a person who is 18 once sex is involved.