What is the oldest a man can father a child?

No limit: No age limit for men especially with modern assisted reproduction technology (artificial insemination etc.).

All depends: If you keep yourself fit and healthy, for a man age is not important. A dad in india fathered his 21st child at age of 94 and one in britain at age 74!be considerate of your child, think about it - is it really fair to have a child at that age ? Remember there are higher risk for chromosome abnormalities, too.