Poisson predictions and constant means

Question

I need to perform a Poisson regression analysis of how number of visits to the doctor in the two week period varies as a function of age group, sex and illness. However, I need to hold sex and number of illnesses constant at their mean values, and then predict rates of visits to a doctor over a two week period for different age groups.

Here is a sample of my data:

visits gender illness age.category

   1 female       1          <30
   1 female       1          <30
   1   male       3          <30
   1   male       1          <30
   1   male       2          <30
   1 female       5          <30
   1 female       4          <30
   1 female       3          <30
   1 female       2          <30
   1   male       1          <30

However, I'm unsure of how to keep sex at its constant mean when making the prediction. Do I need to convert it into numeric first or do something else?

I know how to perform a poisson regression, it's making the prediction that I am struggling with.