Mr. Justice George Strathy, of the Superior Court of Ontario, said the provision violates equality guarantees in the Charter of Rights and Freedoms, and provides no adequate justification for downgrading the contributions of older workers to society.
"A retirement age of 75 ... will assist in attracting qualified and accomplished individuals to the position and will allow them the option of retiring at age 65 or continuing in their roles - making useful and satisfying contributions to their bench and to society - until age 75," he said.
Judge Strathy said it is difficult to understand why JPs were left out in the cold when the province elected in 2005 to abolish mandatory retirement.
"The legislature has confirmed that mandatory retirement is a serious form of age discrimination and has abolished it in the public and private sectors," he said.
"There is no evidence in the record before me of any reasons for the selection of that age - as opposed to any other age - in any of the debates in the legislature at the time, or indeed, of any analysis of this issue on the part of the government, either then or since."
A lawyer for the JPs, Mary Cornish, said in an interview yesterday that JPs who wish to continue working will in future be required to provide a medical certification of fitness each year until they reach age 75.
In an average year, JPs deal with 1.7 million charges under the Provincial Offences Act, including tens of thousands of bail hearings and thousands of search-warrant applications.
The Crown argued that mandatory retirement does not diminish the dignity of JPs, and that it is an important element of preserving judicial independence.