I do not live in California, I do know that minors are not subject to the same laws as adults given they are not of legal age to enter into a contract. While a purchase may not seem like a contract, a purchase is a contract. Think about it this way: the store is providing a good and the customer exchanges money for the good. By exchanging money for goods, one is entering into a contract.
A minor may be able to return purchased items based upon three ideas. First, they are not legally able to agree to a contract. Second, a company cannot assume that a minor can, or will, read and understand the fine print regarding returns. Lastly, a minor can simply change his or her mind without legal repercussions.