My boyfriend just turned 18 and I'm 16. Would it be unlawful If I engage in sexual activity?

Yes. It is unlawful for an adult (18 or older) to engage in sexual intercourse with a minor in the state of California. Age 18 is considered the age of majority and the age a person becomes accountable to the "adult" system of justice. This is true even if the 18 year old is in high school. The legal age to consent to have sexual intercourse in California is 18. The 18 year old risks criminal legal action if engaged in a sexual relationship with a minor. The minor would be considered the victim and not be charged. With a 2 year age difference, if convicted of unlawful sexual intercourse, the most probably classification for the crime would be a "misdemeanor" which can include a fine, civil penalty and up to a year in jail; however, the law states that these crimes can be classified as a misdemeanor or a felony, so it could go either way depending on the circumstance.