It clearly shows a shot of Joe when he is 8 years old and in a trash can in the Grand Canyon. Then when he's recalling when his sister said, "That's why Dad named you Joe Dirt instead of nnnnneeww", it does it twice and on the second time he is an older version of the 8 year old. They obviously forgot that he was left at the Grand Canyon and couldn't possibly be in their car if he was older.