Simplify the following text for a Grade 6 native English-speaking person.
Original: Since 1961 , over 200,000 Americans have joined the Peace Corps and have served in 139 countries . Many former volunteers have risen to national prominence , not least the four who have served as Peace Corps Directors .
Simplified: Since 1960 , more than 190,000 people have served as Peace Corps volunteers in 139 countries .