We had a family member APPLY to be a police officer when he was 20. He entered training at about age 20 and a half, and when training was done he had turned 21 and received his badge. 21 was the minimum age requirement.
Guessing the situation might be similar for you, but trying reaching someone at US Customs https://www.cbp.gov/ with the exact same question----when can I first apply? Good luck