WWII Draft Registration Card

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I'm a little confused. Along with my grandfather Santiago's WWI draft registration card, I also found his WWII draft registration card. He was 56 at the time and living in Los Angeles, California. I was unaware that this is something he (or anyone else for that matter) would have been required to do, especially after already serving in the armed forces. Maybe someone can enlighten me on this. At any rate, it is still a great find.