The department has a long-standing policy that gardaí must retire at 60.
The Government can make exceptions in certain cases, as it did with the current Garda Commissioner, Fachtna Murphy. He was due to retire in June 2007, but was allowed to stay on until December this year.
The policy has been a bone of contention with senior gardaí for years and associations representing superintendents and chief superintendents have challenged it through Government channels and the courts.
Martin Donnellan, who was forced to retire as assistant commissioner in 2008 on age grounds, lost his case in the High Court after the judge ruled that the state was entitled to maintain the retirement age despite EU directives on the area.
His challenge was to a government regulation (Statutory Instrument) in 1996 which lowered the retirement age for assistant commissioners from the age of 65 to 60.
The state argued the lower retirement age was necessary to ensure talented younger people can move through the Garda ranks.
It also claimed that restoring the age to 65 would create a blockage at senior level.
Mr Justice Liam McKechnie said the regulation was introduced as part of a policy aimed at motivating the force, freeing up positions in higher ranks for individuals whose ambition was to progress through the Gardaí and allowing senior management promote particularly talented people earlier than might previously have been expected.
A number of senior officers have lodged complaints with the Equality Authority.
The move by the European Commission follows a complaint it received from the Association of Chief Superintendents.
The association is claiming the policy is discriminatory.
Members of the Garda Reserve are permitted to serve until they are 65.
In Northern Ireland and Britain senior officers are allowed to continue working until 65. In some other European countries the retirement age is 67.
The commission’s employment, social affairs and equal opportunities directorate has formally written to the Government asking for a statement of its policy and the justification for it.
A spokesman for the Department of Justice yesterday said: “A complaint was made by senior gardaí to the European Commission and in the normal course of events with complaints the commission is investigating it.”
He confirmed that department officials were drafting a report: “There will be a response. If the European Commission wants information from the department that will be provided.”
The spokesman pointed out that the department had “justification” for its policy and that it was for “rational reasons”.