Please don't call it statutory rape. The correct term in California is "unlawful intercourse," wiich sounds a lot better.
Unlawful intercourse with a minor more than three years younger, Penal Code 261.5(c), can be punished as a felony with up to three years in prison, or a misdemeanor with up to a year in jail.
Fortunately, that means the DA would have to file charges within three years of the offense. (If she was fifteen or younger, you could have been charged with a violation of Penal Code 288(c), and the DA would have up to her 28th birthday -- and possibly longer -- to charge you.)
It sounds like you're in the clear, but in the future, DON'T CONFESS TO DETECTIVES! If contacted by the police about a crime, politely tell them you do not wish to speak to them, and you will not answer any questions withhout an attorney.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this site, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.
I agree that you should never confess to anything. I disagree that you are probably in the clear. The statute of limitations in many sex cases begins to run from the time of disclosure to law enforcement. If the prosecutor gets cute and charges you with rape, ie., forcible sexual intercourse, then the statute of limitations is 8 years. So, either keep your mouth shut or hire an attorney immediately to run interference for you and make calls and conduct investigation on your behalf.
I handle many sex offense cases and you can contact me through this AVVO website contact information or through my website at www.jameswarden.com.
But, do not just wait around and hope for the best. "Don't hope for victory, plan for it."