Sir Christopher Kelly, chair of the NSPCC and of the Committee on Standards in Public Life, said in a report published last week that Heyday was a "classic case study of what not to do if you want an organisation to succeed".
In February, Age Concern England asked Kelly to investigate Heyday, which offered people approaching retirement a £26-a-year subscription to a website, magazine and products. He had been advising ACE on its merger with Help The Aged. The request followed a highly critical report by the Charity Commission, which said trustees did not show "sufficient oversight and critical challenge".
Kelly's report says "there was a significant failure of governance", and Heyday was "planned and implemented in a way that was almost bound to fail".
It also gives, for the first time, a detailed breakdown of the £22m losses incurred by Age Concern and its trading subsidiary, Acent, over Heyday. They include £6.7m on an IT system commissioned from IBM that did not work properly when Heyday launched in 2006, and £7.3m on employing staff.
"Heyday was a high-profile programme by a major charity that went disastrously wrong and ended up costing the charity significant amounts of money," said Kelly. "Not to account for that in an intelligible way to their own members and supporters, still less the wider public, seems to me to be untenable."
Kelly said he was "struck by an element of defensiveness in the ACE culture, and by what I see as an unwillingness on the part of some, until very recently, to face up to the reality of what happened with Heyday".
He said there was a "strong element of wishful thinking in the charity's approach" and "some of those concerned became carried away with the strength of the vision". There were "potentially important lessons here for other charities" to learn, he added.
He said they included: do not let your head rule your heart; ensure firm evidence; and do not sideline dissent.
Gordon Lishman, director-general at Age Concern, stepped down when the charity merged with Help the Aged in April.
Dianne Jeffrey, chair of the merged charity, said there were "no plans to set up a project like Heyday".