In his ruling Mr Justice Blake said he recognised the "very substantial weight" of arguments put forward by Age Concern, Help the Aged and the Commission for Equality and Human Rights, to stop people being retired at age 65.
But he also took into account the Governments decision earlier in the summer to bring forward a review of default retirement from 2011 to 2010, which will consider whether a default retirement age is still "appropriate and necessary."
Mr Justice Blake commented that a default retirement age would be unlawful if it was introduced in 2009, but his decision to allow it was based on circumstances and evidence at the time.
Commenting on the ruling, Adam Marshall, director of policy at the British Chambers of Commerce (BCC), said: "This was the right decision. The vast majority of businesses value their older employees and the considerable experience that they can bring to a firm. We do not believe there is evidence of widespread use or abuse of the system; only 1 in 4 businesses we surveyed use the default retirement age."
"Employees already have the right to request to postpone their retirement, and we believe the existing rules allow for the fairest outcome on both sides. Businesses need a period of stability to allow all the recent changes to employment legislation to bed down. They don't want more tinkering with employment rules."
This morning's decision means hundreds of retirement-related Employment Tribunal Decisions can now move forward, taking into account the Judge's observations on the legality of the default retirement age of 65 in 2009 when considering the cases.
John Wadham, legal group director for the Equality and Human Rights Commission, said, "The number of older employees is increasing and the law should support those who wish to carry on working and making an economic contribution. Many employers have said they think that having a mandatory retirement age is more trouble than it's worth.
"The judge has sent out a strong signal that it is only a matter of time before the default retirement age of 65 is removed, and we will consider what action we could take next. The Government's promised review has already been brought forward from 2011 to 2010. It could act sooner to strike the DRA out of the rule books using the Equality Bill."
There are 1.4 million people working past state pension age (65 for men, 60 for women), according to the most recent Labour Market figures from the Office of National Statistics. This is up from 1.2 million people the last year.