[R] A model for disease progression

[Damon Wischik]

I would be grateful for advice about the following problem. It's not
directly R-related, but I'm hoping that R will help me analyse the
following data.

I have a table which indicates the progression of a certain age-related
disease.  At a certain point in time, a population was sampled; and I have
measurements for the age of each individual, and their disease stage.
(Disease stage is an ordered factor, none<A<B<C<D.)

I hypothesize that the rate of disease-onset depends on age, but that
(once the disease has started) its rate of progression depends on the
current disease stage and not on age.

I would like to plot the data in a sensible way, to let me see if this
hypothesis is reasonable, and to let me explore possible functional forms
for the two rates.

I can see how to plot the rate of disease onset. I could, for example,
plot for each age the fraction of people who have acquired the disease by
that age. But I don't know how to visualize the rate of disease
progression. 

All suggestions (especially pointers to relevant literature or R packages) 
gratefully received!

Damon.