Answer:
In 49 states, adults (18 years of age and older) cannot legally take part in sexual conduct with minors (17 years of age and under). This is not limited to sexual intercourse, and any police officer who believes this is wrong will try their hardest to find some kind of illegal activity to arrest you for. This can include general things like cuddling, sleeping in the same bed, etc. ("sexual activity").
However, you ask of underage dating. This depends on your definition of dating. You can legally: hold hands, talk, stay the night at each other homes, go to dinner, etc. etc. They cannot arrest two people for going to dinner as friends (never admit to dating). It's anything that goes a step further that is illegal. Same in California as the forty-eight other states with these laws.
Mississippi is the only state where a minor (16+) can participate in sexual activity with an adult.