my bf has a daughter that is 5. the mother was 19-20 when the child was born. Is there a statute of limitations on this since the mother was over age and my bf in question was a minor? If there is a law protecting him, can anything be done to her, the mother?

Yes, possibly.
If a crime occured and was reported to the authorities or the District Attorney, she could have charges filed against her. Determining the charges would depend on the age of the boy-friend at the time the sexual intercourse first occured. It would be up to the district attorney to investigate and determine what crime, if any, was committed; the classification of the crime and whether the statute of limitations would prevent prosecuting the crime.

Source: Arizona State Legislature

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.

13-107. Time limitations

A. A prosecution for any homicide, any offense that is listed in chapter 14 or 35.1 of this title and that is a class 2 felony, any violent sexual assault pursuant to section 13-1423, any violation of section 13-2308.01, any misuse of public monies or a felony involving falsification of public records or any attempt to commit an offense listed in this subsection may be commenced at any time.

B. Except as otherwise provided in this section, prosecutions for other offenses must be commenced within the following periods after actual discovery by the state or the political subdivision having jurisdiction of the offense or discovery by the state or the political subdivision that should have occurred with the exercise of reasonable diligence, whichever first occurs:

1. For a class 2 through a class 6 felony, seven years.

2. For a misdemeanor, one year.

3. For a petty offense, six months.

C. For the purposes of subsection B of this section, a prosecution is commenced when an indictment, information or complaint is filed.

D. The period of limitation does not run during any time when the accused is absent from the state or has no reasonably ascertainable place of abode within the state.

E. The period of limitation does not run for a serious offense as defined in section 13-604 during any time when the identity of the person who commits the offense or offenses is unknown.

F. The time limitation within which a prosecution of a class 6 felony shall commence shall be determined pursuant to subsection B, paragraph 1 of this section, irrespective of whether a court enters a judgment of conviction for or a prosecuting attorney designates the offense as a misdemeanor.

G. If a complaint, indictment or information filed before the period of limitation has expired is dismissed for any reason, a new prosecution may be commenced within six months after the dismissal becomes final even if the period of limitation has expired at the time of the dismissal or will expire within six months of the dismissal.

13-1407. Defenses to prosecution

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.