I think the answer would be executive power.
You have to be 16 to get your drivers license. No younger no older
TRAFFICKING VICTIMS PROTECTION RE AUTHORIZATION ACT OF 2013 provides holistic services for human trafficking survivors, prohibits United States funds from going to any country using child soldiers and imposes reporting compliance requirement on federal agencies. The Act, which is a re- authorization of Trafficking Victim Protection Act of 2000 was put in place to establish a comprehensive approach to human trafficking by creating new criminal offences and to establish protection and assistance for victims.