These two studies arrive at what seems to be a more sensible conclusion:

<A HREF="http://www.jstor.org/stable/2778031">Age and Scientific Performance</A>, Stephen Cole (1976).

> The long-standing belief that age is negatively associated with scientific productivity and creativity is shown to be based upon incorrect
> analysis of data. Analysis of data from a cross-section of academic
> scientists in six different fields indicates that age has a slight
> curvilinear relationship with both quality and quantity of scientific
> output. These results are supported by an analysis of a cohort of
> mathematicians who received their Ph.D.'s between 1947 and 1950.
> There was no decline in the quality of work produced by these
> mathematicians as they progressed through their careers.

<A HREF="http://www.jstor.org/stable/284859">Age and Achievement in Mathematics: A Case-Study in the Sociology of Science</A>, Nancy Stern (1978).

> The claim that younger mathematicians (whether for physiological or
> sociological reasons) are more apt to create important work is
> unsubstantiated... I have found no clear relationship between age and
> achievement in mathematics.

For anecdotes and "advice to aging mathematicians", I might recommend <A HREF="http://link.springer.com/content/pdf/10.1007/BF03026857.pdf">Mathematical menopause, or, a young man's game?,</A> by Reuben Hersh (The Mathematical Intelligencer, 2001).

> Until we find a consensus about which advances are "major," we can't
> refute Hardy's claim that no major advance has been made by a
> mathematician over 50. But his slogan, "Mathematics is a young man's
> game," is misleading, even harmful.