When do the negative effects of boxing begin to appear?

While other injuries repair relatively easily, brain tissue, once damaged, remains damaged. The symptoms of such brain damage - commonly known as being 'punch drunk' - include slurred speech, slow reactions and even occasional blackouts. These symptoms may take years to appear – on average about 16 years after taking up boxing, but sometimes as later as 40 years, long after the boxer has retired from the sport.  
    
      One of the most famous accounts of brain damage being caused by boxing is in that of former heavyweight champion, and the greatest of all time, Muhammad Ali. Muhammad Ali was diagnosed with Parkinson's disease in 1984 at the age of 42. There is a widely held belief that his condition was caused by repeated blows sustained to the head in his illustrious career. (Macnair)

     Ali began showing symptoms of the disease soon after retiring from the boxing ring in 1981. But his condition was not diagnosed until three years later. By that stage he had developed tremors, his speech was slurred, and his body movements had become slow. Doctors prescribed drugs which alleviated the worst of his symptoms.

     Ali is no longer able to speak in public ( "Muhammad Ali and," 2009)