SAS: make values missing - sas

Question

I am trying to make some existing values to missing values (not deleting them).
Here is the basic structure of my data set.
I want to treat AGE and GENDER as missing whenever A is less than B. For example, when A=1 and B=3, I want to treat values of AGE and GENDER on the last two rows as missing (as shown on the data sets).
In my data both A and B go from 1 to 4 and have every combination of them.
Asterisks mean I have more data between them. Thanks in advance!
BEFORE
ID A B AGE GENDER
--------------
1 1 1 35 M
* * * * *
* * * * *
5 1 2 23 F
5 1 2 21 M
6 1 2 42 F
6 1 2 43 M
* * * * *
* * * * *
20 1 3 43 F
20 1 3 39 M
20 1 3 23 M
21 1 3 32 F
21 1 3 39 M
21 1 3 23 F
* * * * *
* * * * *
55 2 4 32 M
55 2 4 12 M
55 2 4 31 F
55 2 4 43 M
* * * * *
* * * * *
AFTER
ID A B AGE GENDER
--------------
1 1 1 35 M
* * * * *
* * * * *
5 1 2 23 F
5 1 2 . .
6 1 2 42 F
6 1 2 . .
* * * * *
* * * * *
20 1 3 43 F
20 1 3 . .
20 1 3 . .
21 1 3 32 F
21 1 3 . .
21 1 3 . .
* * * * *
* * * * *
55 2 4 32 M
55 2 4 12 M
55 2 4 . .
55 2 4 . .
* * * * *
* * * * *

Answer 1

How about now?
data temp;
retain idcount 0;
set olddata;
** Create an observation counter for each id **;
prev_id = lag(id);
if id ^= prev_id then idcount = 0;
idcount = idcount + 1;
run;
** Sort the obs by ID in reverse order **;
proc sort data=temp;
by id descending idcount;
run;
data temp2;
retain misscount 0;
set temp;
by id descending idcount;
** Keep the previous age and gender **;
old_age = age;
old_gender = gender;
** Count the number that should be missing **;
if a < b then nummiss = b - a;
else nummiss = 0;
** Set a counter of obs that we will set to missing **;
if first.id then misscount = 0;
** Set the appropriate number of rows to missing and update the counter **;
if misscount < nummiss then do;
misscount = misscount + 1;
call missing(age, gender);
end;
run;
proc sort data=temp2 out=temp3(drop=misscount nummiss idcount prev_id);
by id idcount;
run;