Federal law requires nearly all male U.S. citizens and male immigrants to register at age 18.
It’s important to know that even though he is registered, a man will not automatically be inducted into the military. In a crisis requiring a draft, men would be called in a sequence determined by random lottery number and year of birth. Then, they would be examined for mental, physical, and moral fitness by the military before being deferred or exempted from military service or inducted into the Armed Forces.
Please see this link for answers to frequently asked questions: Frequently Asked Questions: Selective Service System