Walsh in Guide to Surviving Prostate Cancer provides approximately the
following explanation.  As one ages, the nerves which are neceesary for
erections tend to degenerate.   Up to a certain point, this doesn't
matter, but at some point it creates some problems.  A radical
prostatectomy, even if the nerves are spared, does produce some trauma
in the area and the process of nerve degeneration may be accelerated.
Both younger and older men lose some of what is needed, but in the
younger men there is still plenty left over.

The figures are quite startling.  Surgeons like Walsh can preserve
potency in younger men in 80 percent of more of the cases.  For men over
70, the success rate drops to something like 30 percent.

Radiation also affects potency but in somewhat different ways.  Usually
there is not much of a problem immediately, but it may develop over the
next couple of years or more.  This could be due to latent damage to the
nerves or to the circulation in the area.  Men over 70 definitely do
better in this area with radiation, though.

;-}  'cause they is younger?

It is defintely better for younger people, but it is probably not the most
important factor.  Capsule penetration, skill of the doctor, and sexiness of
the wife are much more important.

.... okay, maybe not the latter.