[R] survfit using quantiles to group age

[Eleni Rapsomaniki]

I am using the package Design for survival analysis. I want to plot a  
simple Kaplan-Meier fit of survival vs. age, with age grouped as  
quantiles. I can do this:

survplot(survfit(Surv(time,status) ~ cut(age,3), data=veteran)

but I would like to do something like this:

survplot(survfit(Surv(time,status) ~ quantile(age,3), data=veteran)  
#will not work

ideally I would like to superimpose estimates from cph models, which  
automatically fit the 2nd to 4rth quantiles for age, so I need the age  
groups to be grouped the same.

Any help greatly appreciated!
Eleni Rapsomaniki