data b(keep=age sum);
set a;
retain sum;
if (mod(age,5)=1) then sum=n;
else sum+n;
if (mod(age,5)=0);
run;

proc sql;
    create table b as (
    select distinct floor(age/5) as age_group, sum(n) as n
    from a
    group by floor(age/5))
    ;
quit;

如果age有缺失的话在mod(age,5)这里会出现bug，比如缺少age=5时则第二个年龄组没有输出

You are right. Here is the updated code.

data a;
set a;
g=floor(age/5);
run;
data b;
set a;
retain sum;
by g;
if first.g then sum=n;
else sum+n;
if last.g;
run;

SAS has many useful procedures. Here is another way to do it with proc format and proc means.

proc format;
value agerange
0='0'
1-5='1-5'
6-10='6-10'
11-15='11-15'
;
run;
proc means data=a sum noprint ;
var n;
by age;
format age agerange.;
output out=c sum=sum;
run;

谢谢你，但是你的后一种方法我试了一下，发现0岁组丢掉了，而且计算的合计值好像也不太对呢！