The Guinness editors do their best to stay away from politics , though we do learn that Iran has the '' youngest voting age '' -LRB- 15-year-olds can cast ballots -RRB- , that Fidel Castro is the '' longest serving head of state '' and that Italy 's Silvio Berlusconi is the '' richest prime minister , '' having amassed '' a personal wealth of $ 10 billion , according to Forbes . ''