No this is not 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.

Is it illegal for a minor to date an 18 year old if they were dating before he turned 18?

Any sexual activity with a person under 18, even if both are under 18, is illegal. There is no exception for sexual relationships hat began before one of the parties was 18. Its still illegal

Can you date an 18 year old if youre a minor?

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.