I don't see anything in there that prohibits selling gift cards to minors of any age.
My guess is that the store people just don't want to take any chances selling to a minor because a minor is allow by law to disavow contracts.
If your brother wants to make an issue of it he should ask "What law? Please cite me a statute number so I can look it up."
If the person can't comply then he's probably clueless and your brother's option is to go elsewhere and buy the card or just pay cash for whatever he wants to buy at whatever store he wants the card for.