Page "Ford Prefect (character)" Paragraph 6

Ford is approximately two hundred years old, as supported by the books.

When, in the first novel, Zaphod steals the Heart of Gold, it is on his ( Zaphod's ) two hundredth birthday.

It is later mentioned that Ford and Zaphod attended school together, even having some of the same classes, which would indicate that they are roughly the same age.