Age as a time dependant covariate in Cox regression

Question

I have a question regarding time-dependant variables in survival analysis. Do you usually count age as a time varying variable? I am looking at a population of cancer patients who received certain treatment and were cured initially. They were then followed up for certain period of time. The total follow up time is up to 15 years, so it is relatively long, but some have of course a much shorter follow up time. The event of interest is whether they developed recurrence of their cancer or not. So in SAS, here is how I am doing this

proc phreg data=have ;
Title 'Cox for cancer recurrence';
class sex tumor_differentiation;
model Time*Recurrence(0)=   age sex tumor_size tumor_differentiation/rl;
run;

And in R

surv_object <- Surv(time = df$Time, event = df$Recurrence)
fit.coxph <- coxph(surv_object ~ Age +Sex + TumorSize + TumorDifferentiation, 
                   data = have)

The question here is, would you put in Age as a time dependant co-variate or would you just put in age at baseline in your model? Thank you for your insights and appreciate your help

Answer 1

In the cancer research, the age at diagnosis has clinical importance, so in the usual setting, we use age at diagnosis as a variable, and it is fixed!