Without a work permit, a person must be at least 18 years old. I got this from Maryland’s Department of Labor website:

Work permits are required for all minors under the age of 18 to work in the State of Maryland.
Work permits are not issued to minors under the age of 14.
Work permits are issued by Issuing Officers in Maryland secondary schools, by some government sponsored summer jobs programs, by the Ocean City Police Department and by the Division of Labor and Industry. Minors and their parents are encouraged to contact their local school to obtain a work permit.
To obtain a Work Permit: Work permits are not issued until the minor has been offered a job. The work permit allows the minor to perform certain duties for a particular employer. If the minor changes jobs, a new work permit must be obtained. The work permit form is obtained from the school, the minor completes some basic information and signs the permit. The minor’s parent or guardian signs the permit. The work permit is taken to the employer who enters their name and address and describes the minor’s duties. The employer signs the permit. The work permit is then taken to the Issuing Officer. Issuing Officers are required to verify the minor’s date of birth. If the minor’s date of birth is not on school records, then the minor must present proof of age. Acceptable documents include a birth certificate, motor vehicle identification, passport, baptismal certificate or other government document that verifies the date of birth. The validated work permit, with the Issuing Officer’s signature must be in the employer’s possession before the minor is permitted to work.