[R] Link two variables

[Michele Grassi]

Hi.
How can i link two variables, so when i group the first 
i group also the second one?
I try whit create a model matrix (model.matrix) and 
use  a grouping function like "balancedGrouped(nlme)".
More specific i have varx:AGE and vary(binomial):o
(dead),1(alive). I want to create age group,link vary 
group, compute the mean of x link the mean of y.
Thank you.
Michele.