proc sort data=m;
by age;
if first.age then count=1;
count=count+1;
run;
data target;
set m;
if count=x;
run;

非常感谢，这一段很有启发意义，尤其是那个count，这段还有一点点小问题，sort下面的if不能被执行，受你启发，我写了一个新的算法可以解决这个问题，但有点冗杂，尚需优化。
data m;
set sashelp.class;
run;

data a;
do id= 1 to 19;
output;
end;
run;

data aa;
merge a m;
run;

proc sort data=aa out=aaa;
by age;
run;

proc rank data=aaa out=aaaa;
var id;
ranks count;
by age;
run;

data b;
set aaaa;
if count=X then output;
run;

first unintialized 这种情况时有发生，目前还没有找到原因。