Statutory rape is defined by Penal Code Section 261.5 as unlawful sexual intercourse with a person who is not the defendant's spouse if that person is under the age of 18 years old.  If the age difference is only three years then the defendant is only guilty of a misdemeanor.  If the defendant is more than three years older than the victim then the crime is a wobbler and can be either a misdemeanor or a felony.  Anyone over 21 who has sexual intercourse with someone under 16 can face 2, 3 or 4 years in a prison.  In general statutory rape convictions don’t require registration under Penal Code 290.