I am now 21 and my ex-boyfriend is 18. He lied and told me he was 18 when I was 20 and we had sex. Now that he is 18, can he press charges against me for statutory rape? And if he presses charges, will I go to jail even though he is now 18 years old?

The possiblity of charges always exists if there has been a violation of 261.5.

In order for anything to happen, someone must first file a complaint against you with police or the DA's office. The police or DA, make a decisions based on the complaint whether to investigate the matter further or not. If an investigation is opened, the decision to file charges and pursue a case rests with the DA's office.

Whether the DA seeks a conviction of a misdemeanor or a felony is determined on a case by case basis. Given the age difference in your situation, according to the statute this could be charged as either felony or misdemeanor. A local attorney who specializes in these cases would be very insightful as to how the DA determines the class of crime.

Statute of Limitations laws - Criminal Prosecutions

Citation: Penal Code §§ 799; 800; 801; 803(f)(1) & (g)(1)

The statute of limitations for certain offenses is a period within 1 year after a victim under 18 years of age reports to a responsible adult or agency that he or she is a victim of an offense listed below:

The statute of limitations for the following offenses is a period within 1 year after a victim (of any age) reports to a California law enforcement agency that he or she, while under 18 years of age, was a victim of any of those offenses.

• Rape
• Sodomy
• Continuous sexual abuse of a child

Generally, the following statutes of limitations apply:

• For an offense punishable by life imprisonment, there is no statute of limitations
• For an offense punishable by imprisonment for 8 years or more, a period within 6 years after commission of the offense
• For all other offenses subject to a term of imprisonment, a period within 3 years after commission of the offense.
• For Misdemeanors, a period of one year after the commission of the offense.

Civil Actions

Citation: Code of Civ. Pro. § 340.1(a)

The statute of limitations for filing a claim based on childhood sexual abuse extends for a period within 8 years after the minor reaches 18 years of age or a period within 3 years after the minor discovered or reasonably should have discovered that his or her psychological injury or illness occurring after 18 years of age was caused by the sexual abuse.

Sources:
http://www.childwelfare.gov/index.cfm
http://www.leginfo.ca.gov/calaw.html