Your grandson is incorrect. If a complaint is filed, he can be charged with a criminal sexual offense 13-1405 Sexual conduct with a minor. Even as a minor himself, charges can be filed. It is not legal for minor's to have sex with minors in the state of Arizona. Being 18 years of age now, he could be charged with a sexual offense. There are legal affirmative defenses to this crime however, the description you gave does not fit the definitions outlined in the law. See below.

13-1405. Sexual conduct with a minor; classifications

A. A person commits sexual conduct with a minor by intentionally or knowingly engaging in sexual intercourse or oral sexual contact with any person who is under eighteen years of age. B. Sexual conduct with a minor who is under fifteen years of age is a class 2 felony and is punishable pursuant to section 13-604.01. Sexual conduct with a minor who is at least fifteen years of age is a class 6 felony. Sexual conduct with a minor who is at least fifteen years of age is a class 2 felony if the person is the minor's parent, stepparent, adoptive parent, legal guardian or foster parent and the convicted person is not eligible for suspension of sentence, probation, pardon or release from confinement on any basis except as specifically authorized by section 31-233, subsection A or B until the sentence imposed has been served or commuted.

13-1407. Defenses

It is a defense to a prosecution pursuant to sections 13-1404 and 13-1405 involving a minor if the act was done in furtherance of lawful medical practice. B. It is a defense to a prosecution pursuant to sections 13-1404 and 13-1405 in which the victim's lack of consent is based on incapacity to consent because the victim was fifteen, sixteen or seventeen years of age if at the time the defendant engaged in the conduct constituting the offense the defendant did not know and could not reasonably have known the age of the victim. C. It is a defense to a prosecution pursuant to section 13-1402, 13-1404, 13-1405 or 13-1406 if the act was done by a duly licensed physician or registered nurse or a person acting under the physician's or nurse's direction, or any other person who renders emergency care at the scene of an emergency occurrence, the act consisted of administering a recognized and lawful form of treatment that was reasonably adapted to promoting the physical or mental health of the patient and the treatment was administered in an emergency when the duly licensed physician or registered nurse or a person acting under the physician's or nurse's direction, or any other person rendering emergency care at the scene of an emergency occurrence, reasonably believed that no one competent to consent could be consulted and that a reasonable person, wishing to safeguard the welfare of the patient, would consent. D. It is a defense to a prosecution pursuant to section 13-1404 or 13-1405 that the person was the spouse of the other person at the time of commission of the act. It is not a defense to a prosecution pursuant to section 13-1406 that the defendant was the spouse of the victim at the time of commission of the act. E. It is a defense to a prosecution pursuant to section 13-1404 or 13-1410 that the defendant was not motivated by a sexual interest. It is a defense to a prosecution pursuant to section 13-1404 involving a victim under fifteen years of age that the defendant was not motivated by a sexual interest. F. It is a defense to a prosecution pursuant to section 13-1405 if the victim is fifteen, sixteen or seventeen years of age, the defendant is under nineteen years of age or attending high school and is no more than twenty-four months older than the victim and the conduct is consensual.

If you are being charged in this matter, I strongly advise you seek the advice of an attorney; if your income is such that you are unable to obtain representation, you may ask the court to appoint counsel. The minor's true age will be a big factor in your defense. If there is a pregnancy and you are uncertain you are the father, a paternity test will be required. I've outlined the laws below that may be relevant to your situation. Please read them carefully.

13-1405. Sexual conduct with a minor; classifications

A. A person commits sexual conduct with a minor by intentionally or knowingly engaging in sexual intercourse or oral sexual contact with any person who is under eighteen years of age. B. Sexual conduct with a minor who is under fifteen years of age is a class 2 felony and is punishable pursuant to section 13-604.01. Sexual conduct with a minor who is at least fifteen years of age is a class 6 felony. Sexual conduct with a minor who is at least fifteen years of age is a class 2 felony if the person is the minor's parent, stepparent, adoptive parent, legal guardian or foster parent and the convicted person is not eligible for suspension of sentence, probation, pardon or release from confinement on any basis except as specifically authorized by section 31-233, subsection A or B until the sentence imposed has been served or commuted.

13-1421. Evidence relating to victim's chastity; pretrial hearing

A. Evidence relating to a victim's reputation for chastity and opinion evidence relating to a victim's chastity are not admissible in any prosecution for any offense in this chapter. Evidence of specific instances of the victim's prior sexual conduct may be admitted only if a judge finds the evidence is relevant and is material to a fact in issue in the case and that the inflammatory or prejudicial nature of the evidence does not outweigh the probative value of the evidence, and if the evidence is one of the following: 1. Evidence of the victim's past sexual conduct with the defendant. 2. Evidence of specific instances of sexual activity showing the source or origin of semen, pregnancy, disease or trauma. 3. Evidence that supports a claim that the victim has a motive in accusing the defendant of the crime. 4. Evidence offered for the purpose of impeachment when the prosecutor puts the victim's prior sexual conduct in issue. 5. Evidence of false allegations of sexual misconduct made by the victim against others. B. Evidence described in subsection A shall not be referred to in any statements to a jury or introduced at trial without a court order after a hearing on written motions is held to determine the admissibility of the evidence. If new information is discovered during the course of the trial that may make the evidence described in subsection A admissible, the court may hold a hearing to determine the admissibility of the evidence under subsection A. The standard for admissibility of evidence under subsection A is by clear and convincing evidence.

13-1407. Defenses

A. It is a defense to a prosecution pursuant to sections 13-1404 and 13-1405 involving a minor if the act was done in furtherance of lawful medical practice. B. It is a defense to a prosecution pursuant to sections 13-1404 and 13-1405 in which the victim's lack of consent is based on incapacity to consent because the victim was fifteen, sixteen or seventeen years of age if at the time the defendant engaged in the conduct constituting the offense the defendant did not know and could not reasonably have known the age of the victim. C. It is a defense to a prosecution pursuant to section 13-1402, 13-1404, 13-1405 or 13-1406 if the act was done by a duly licensed physician or registered nurse or a person acting under the physician's or nurse's direction, or any other person who renders emergency care at the scene of an emergency occurrence, the act consisted of administering a recognized and lawful form of treatment that was reasonably adapted to promoting the physical or mental health of the patient and the treatment was administered in an emergency when the duly licensed physician or registered nurse or a person acting under the physician's or nurse's direction, or any other person rendering emergency care at the scene of an emergency occurrence, reasonably believed that no one competent to consent could be consulted and that a reasonable person, wishing to safeguard the welfare of the patient, would consent. D. It is a defense to a prosecution pursuant to section 13-1404 or 13-1405 that the person was the spouse of the other person at the time of commission of the act. It is not a defense to a prosecution pursuant to section 13-1406 that the defendant was the spouse of the victim at the time of commission of the act. E. It is a defense to a prosecution pursuant to section 13-1404 or 13-1410 that the defendant was not motivated by a sexual interest. It is a defense to a prosecution pursuant to section 13-1404 involving a victim under fifteen years of age that the defendant was not motivated by a sexual interest. F. It is a defense to a prosecution pursuant to section 13-1405 if the victim is fifteen, sixteen or seventeen years of age, the defendant is under nineteen years of age or attending high school and is no more than twenty-four months older than the victim and the conduct is consensual. Read this article: "The Private Attorney".