You're friend is mistaken regarding Iowa law, while it does not specifically address the age issue in it's text regarding honoring the permits of other states having a permit does not trump the prohibition on persons under 21 being in possession of a handgun and not language in the Iowa code treats a permit asa get out of jail free card regarding age.
IE the permit is what keeps you from being charged for specific issues, carrying under 21 is not one of them.
The IA Department of Public Safety sates the following regarding honoring other state's permits.
In Iowa one must be 21 to apply for a non-professional permit to carry weapons, this everyone carrying under a non-pro permit must be 21.QUESTION: I live in another state. Will Iowa honor my concealed weapon permit issued in another state?
ANSWER: Iowa will honor any valid carry permit issued by any other state and will grant all privileges to such permit holders as those granted to Iowa residents including the concealed or open carrying of a firearm (excluding those classified by Iowa law as offensive weapons (federal NFA or Class 3)) and the concealed carrying of other non-firearm dangerous weapons such as knives with blades in excess of five inches, switchblade knives, Tasers/stun guns, or any other dangerous weapon. Non-firearm dangerous weapons may be carried openly without a permit. Non-firearm dangerous weapons may also be regulated by local ordinance that is more stringent than Iowa law. You do not have to be a resident of the state from which your permit was issued. However, an Iowa resident may only carry with an Iowa issued permit.
724.8 Persons ineligible for permit to carry weapons.
No professional or nonprofessional permit to carry weapons shall be issued to a person who is subject to any of the following:
1. Is less than eighteen years of age for a professional permit or less than twenty-one years of age for a nonprofessional permit.
2. Is addicted to the use of alcohol.
3. Probable cause exists to believe, based upon documented specific actions of the person, where at least one of the actions occurred within two years immediately preceding the date of the permit application, that the person is likely to use a weapon unlawfully or in such other manner as would endanger the person's self or others.
4. Is subject to the provisions of section 724.26.
5. Has, within the previous three years, been convicted of any serious or aggravated misdemeanor defined in chapter 708 not involving the use of a firearm or explosive.
6. Is prohibited by federal law from shipping, transporting, possessing, or receiving a firearm.
[C79, 81, §724.8]
2010 Acts, ch 1178, §6, 19
724.22 Persons under twenty-one — sale, loan, gift, making available — possession.
1. Except as provided in subsection 3, a person who sells, loans, gives, or makes available a rifle or shotgun or ammunition for a rifle or shotgun to a minor commits a serious misdemeanor for a first offense and a class "D" felony for second and subsequent offenses.
2. Except as provided in subsections 4 and 5, a person who sells, loans, gives, or makes available a pistol or revolver or ammunition for a pistol or revolver to a person below the age of twenty-one commits a serious misdemeanor for a first offense and a class "D" felony for second and subsequent offenses.
3. A parent, guardian, spouse who is eighteen years of age or older, or another with the express consent of the minor's parent or guardian or spouse who is eighteen years of age or older may allow a minor to possess a rifle or shotgun or the ammunition therefor which may be lawfully used.
4. A person eighteen, nineteen, or twenty years of age may possess a firearm and the ammunition therefor while on military duty or while a peace officer, security guard or correctional officer, when such duty requires the possession of such a weapon or while the person receives instruction in the proper use thereof from an instructor who is twenty-one years of age or older.
5. A parent or guardian or spouse who is twenty-one years of age or older, of a person fourteen years of age but less than twenty-one may allow the person to possess a pistol or revolver or the ammunition therefor for any lawful purpose while under the direct supervision of the parent or guardian or spouse who is twenty-one years of age or older, or while the person receives instruction in the proper use thereof from an instructor twenty-one years of age or older, with the consent of such parent, guardian or spouse.
6. For the purposes of this section, caliber .22 rimfire ammunition shall be deemed to be rifle ammunition.
7. It shall be unlawful for any person to store or leave a loaded firearm which is not secured by a trigger lock mechanism, placed in a securely locked box or container, or placed in some other location which a reasonable person would believe to be secure from a minor under the age of fourteen years, if such person knows or has reason to believe that a minor under the age of fourteen years is likely to gain access to the firearm without the lawful permission of the minor's parent, guardian, or person having charge of the minor, the minor lawfully gains access to the firearm without the consent of the minor's parent, guardian, or person having charge of the minor, and the minor exhibits the firearm in a public place in an unlawful manner, or uses the firearm unlawfully to cause injury or death to a person. This subsection does not apply if the minor obtains the firearm as a result of an unlawful entry by any person. A violation of this subsection is punishable as a serious misdemeanor.
[C97, §5004; C24, 27, 31, 35, 39, §12958; C46, 50, 54, 58, 62, 66, 71, 73, 75, 77, §695.26; C79, 81, §724.22]
90 Acts, ch 1147, §7; 94 Acts, ch 1023, §119; 94 Acts, ch 1172, §56