Heyday, which has absorbed £5m of Age Concern funds and is struggling to reach membership targets it set when it was established, provides lifestyle information to help people avoid problems in later life.
But when Age Concern applied to use Heyday as a working name of the charity, the commission warned that its activities "could be said to fall outside the object of the charity".
The letter, which has been seen by Third Sector, says that Age Concern's charitable objectives relate to "the elderly" and that Heyday's target group, the over-50s, would not qualify.
It adds that the commission and the courts have never tested whether Heyday's preventive approach falls within the charitable object of the relief of the elderly.
It also warns that private benefits enjoyed by Heyday members might make it "inappropriate" for it to be part of Age Concern.
The letter says it is also unclear whether Heyday's 45,000 members have voting rights in Age Concern. "Uncertainty such as this has led to challenges as to the validity of trustee appointments or changes to the constitution," it says.
Written in January this year by Daphne Young, senior liaison officer at the commission's large charities unit, to Bill Prouse, solicitor and legal adviser for Age Concern, the letter says the charity should revise its objects as part of a current governance review, but does not say this would solve the questions about Heyday.
It adds: "We understand there will be a shift in the marketing of Heyday going forward, as it has been recognised that the right balance was not achieved in early marketing material, giving Heyday the appearance of a private membership organisation."
A spokesman for Age Concern England said: "Age Concern sees it as part of our role as a large charity to innovate and act creatively in carrying out our charitable objectives. It remains our view that the core purpose of Heyday is charitable and that it is entirely appropriate for this work to be within the charity.
"Age Concern is engaged in a constructive dialogue with the commission about ways to clarify and modernise our objectives in line with the new Charities Act.
"This revision of our objectives should help to make it clearer how Heyday's core activities are charitable and should be done by the charity rather than our trading company."
Heyday was launched last May with a target of gaining 300,000 members in its first year, but currently has only 45,000.
One senior manager and two board members have left over the affair, and 34 Heyday staff have lost their jobs following a revision of the business plan and recruiting methods.
An internal Age Concern document seen by Third Sector says that expenditure to date on Heyday totals nearly £10m. Other estimates have put the figure at as much as £16m.