I'm 26years old and I was adopted when I was 6 months old from So. Korea by U.S Citizens. I'd like to know if I should file a N-400 since I'm an adult now or should I use the N-600? The instructions form (N-600) is a little confusing. It seems like I should use that form but the only problem is that I was 19 years old when the child citizenship act was passed in 2-2001. Please help?

You've become US citizen since the time of adoption. All you need is file N-600 to obtain Certificate of Citizenship.