Simplify the following text for a Grade 6 native English-speaking person.
Original: Muhammad Ali ( born Cassius Marcellus Clay , Jr. ; January 17 , 1942 ) is a former American boxer and three-time World Heavyweight Champion , who is widely considered one of the greatest heavyweight championship boxers .
Simplified: Muhammad Ali ( born Cassius Marcellus Clay , Jr. on January 17 , 1942 ) was an American boxer .