 
                            
                        
                        
                        
                                        
                                        Though Erdős used  this remark, it is said to have originated with his friend Stanisław Ulam, as reported in The Man Who Loved Only Numbers : The Story of Paul Erdős and the Search for Mathematical Truth (1998) by Paul Hoffman 
Variants: 
The first sign of senility is when a man forgets his theorems. The second sign is when he forgets to zip up. The third sign is when he forgets to zip down. 
As quoted in Wonders of Numbers : Adventures in Mathematics, Mind, and Meaning (2002) by Clifford A. Pickover, p. 64 
There are three signs of senility. The first sign is that a man forgets his theorems. The second sign is that he forgets to zip up. The third sign is that he forgets to zip down. 
Misattributed
                                    
 
        
     
                             
                             
                             
                             
                             
                            