All male U.S. citizens must register with the Selective Service System within 30 days of his 18th birthday. Selective Service may accept a late registration, but not after a man has reached his 26th birthday.  Male immigrants living in the U.S. between the ages of 18 and 25 must also register, regardless of their legal right to be in the country. For purposes of Selective Service registration, determination of who is “male” is based on gender at birth, not gender identity or gender modified by gender reassignment surgery.