Ciao Here is the example using Stata data.

Code:
use http://www.stata-press.com/data/r13/drugtr
 stset studytime, failure(died)
stcox drug age
Now if we want to get HR risk for subjects less than 56 or more than 56 we make new variable
Code:
gen lessthan56 = age
recode lessthan56 (min/55 = 0)
recode lessthan56 (56/max = 1)
But how is it possible to get HR risk for these two groups?