What would happen if a girl lies about her age?

In many States, including Michigan, misrepresentation of age is not a defense. If they had a fake ID even and they turn out to be underage then that is not a defense and the person can be found guilty of statutory rape. This carries mandatory prison time in Michigan. That person needs a good attorney.

I am guessing that you had sexual relations with an underage girl. If that is the case, her lying about her age does not help you defense because, being underage, should could not legally consent to having sexual relations with you. get a criminal lawyer

It's tough to say whether he has a "good chance" at beating the case or not, because there are many other factors to consider. Based on the limited amount of information you've posted here, it sounds like he has a strong defense. But if, just by appearance, she is clearly underage, then the case may be more challenging.

I am assuming from your question that the charges may be statutory rape. In California, statutory rape is codified in California Penal Code Section 261.5. This reads as follows: 261.5. (a) Unlawful sexual intercourse is an act of sexual intercourse accomplished with a person who is not the spouse of the perpetrator, if the person is a minor. For the purposes of this section, a "minor" is a person under the age of 18 years and an "adult" is a person who is at least 18 years of age. (b) Any person who engages in an act of unlawful sexual intercourse with a minor who is not more than three years older or three years younger than the perpetrator, is guilty of a misdemeanor. (c) Any person who engages in an act of unlawful sexual intercourse with a minor who is more than three years younger than the perpetrator is guilty of either a misdemeanor or a felony, and shall be punished by imprisonment in a county jail not exceeding one year, or by imprisonment pursuant to subdivision (h) of Section 1170. (d) Any person 21 years of age or older who engages in an act of unlawful sexual intercourse with a minor who is under 16 years of age is guilty of either a misdemeanor or a felony, and shall be punished by imprisonment in a county jail not exceeding one year, or by imprisonment pursuant to subdivision (h) of Section 1170 for two, three, or four years. In order to convict you of statutory rape under California Penal Code 261.5, the prosecutor must prove that a male and female engaged in an act of sexual, that the persons involved in the act were not married to each other at the time, and that the alleged victim was under 18 at the time of the offense. If you can prove that you honestly and reasonably believed that the alleged victim was over 18 at the time you had sex, you cannot be convicted under California statutory rape law. The types of evidence that can support your claim could include, for example: Statements made by the alleged victim that he/she was over the age of 18, His/her attire and general appearance, and Where you met the alleged victim (at an adult party or venue, for example).

You have not told me what the charge is. If the defendant is charged with statutory rape then it does not matter what age she says she is, it is rape if you have intercourse with a girl under 17 no matter what she says.

You get charged with sexual battery is what would happen. The state might have a hard time proving guilt depending upon what the facts actually are.

You leave out a very important fact. Am I to assume that the defendant had sex with this underage girl??? If so, the fact that she lied is not a defense. If you had sex and she was 14 (even though she may have said 19), you have committed statutory rape.

Mistake of fact is a defense but not if the reasonable person would have known the girl was about 14 not 19.

That's no defense. Statutory rape is rape, regardless of what the victim says about her age.