Notwithstanding any other provision of local law, no person under the age of twenty-one shall be granted a permit or license to purchase and possess a firearm. If the applicant for a permit or license is a partnership or corporation, only those members of the partnership or corporation over the age of twenty-one may apply for a permit or license to purchase and possess a firearm on behalf of the partnership or corporation. This section shall not apply to any person under the age of twenty-one who has been issued a valid permit or license to possess a firearm on the date that this section shall become law.