In the United States, most males are required to register with the Selective Service System within 30 days of their 18th birthday. and not required after their 25th birthday if you came to the country after the age of 25 you simply tell them that you where not eligible. if you where eligible and didn’t know that you needed to register be honest and tell them what it is and why you didn’t. 1- maybe because you didn’t know that it was a requirement! 2- you never heard of it! 3- you have never done anything that asked you about it in the past be careful if you have applied for student aid before the age of 25 it’s a requirement to to register and they will ask you to register.