The American boxer retired in 2005 , but he still reigns as the youngest heavyweight champion of all time at 20 years of age . The 53 year old was asked by fans online , on how he would do against Tyson Fury and Anthony Joshua. Tyson replied and said , “they are really good fighters and I respect all their skills , it would definitely be very tough fighting them . But if I was in 20 years old and I went against them , I will knock both boxers out. But Mohammed Ali is a different type of boxer , and I don’t think I would have one against him , even in my prime .