9. Rocky Marciano
Retirement Age (Career): 32 (1948-1955)
At 31 and with a 49-0 record (43 KOs), Marciano gracefully bowed out. The original "Rock" was a brawler's brawler. His right hand rearranged mugs (peep the photo, son!) and left fans (and opponents) screaming in the process. Marciano was Arturo Gatti with Mike Tyson power, a straight animal in the ring. Still young and functioning, he didn't want to overstay his welcome like those before him. His family convinced him to retire so he could spend more time with them. We ain't mad at that. Sidebar: So is this Pacquaio v. Mayweather thing FINALLY going to happen?! Till then, MMA>>>boxing.