When Cathy and Maria are discussing Blisters age, they forgot the pregnancy time. Buster was in jail for 18 months, 8 years ago. So he got the woman pregnant 9 and a half years ago. After a 10 month pregnancy, that means the kid was born 8 years and 8 months ago. So when Blister tells Cathy he is 8, that fits the time-line.