if _n_=1 then set

If a user wants to add single obeservation of one dataset to each record of the other dataset, then the user can make use of _n_=1.

%let Final_Year=2012;
%let Final_Week=4;

/*get the last date from date dataset based on input parameters (to calculate the age of a store)*/
data lday(rename=(wk_end_d=lday));
set acct_date(where=(yr_i=&Final_Year and wk_i=&Final_Week));
run;

/*add the last date to each record of store to calculate the age*/
data store_lday;
if _n_=1 then set lday(keep=lday);
set store;
***** age *****;
if open_date ne . then do;
age_in_wks=intck('week',open_date,lday);
age_in_mth=intck('month',open_date,lday) - (day(lday) < day(open_date));
end;
run;