If a draft starts the year you turn 20, you're SOL. You'll be called first until there is nobody left who is turning 20. That's your hold-your-breath year to be probably called up on somebody else's facist agenda and participate in national offense as opposed to defense.
Once every able bodies 20 year old are called up, then it'll be 21, 22,+ in that order, till 26 and then 18 and 19. If a year passes, then it resets to the new 20 year old turners.
If you're out of the US, I dunno how they do conscription in your country. Maybe you can enlighten the rest of us on how your country's conscription system works.