Simplify the following text for a Grade 7 native English-speaking person.
Original: It is one of the most affluent counties in the USA , and has the highest percentage ( 29.2 % ) of residents over 25 years of age who hold post-graduate degrees .
Simplified: It is one of the richest counties in the United States , and has the highest percentage ( 29.2 % ) of residents over 25 years old who hold a post-graduate degree .