Can an 18 year old kiss a 16 year old California?

This is false. Under section 647.6 of the California Penal Code it is a crime for any person to “annoy or molest any child under 18 years of age.”

Is it bad for an 18 year old to date a 15?

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.