How do you choose between character or numeric variables?
Could TYPE be a numeric variable? Will this code work?

DATA MIXED;  
   INPUT @20 TYPE 1. @;  
   IF TYPE = 1 THEN  
      INPUT ID     1-3  
            AGE    4-5  
            WEIGHT 6-8;  
   ELSE IF TYPE = 2 THEN  
      INPUT ID     1-3  
            AGE    10-11  
            WEIGHT 15-17;  
DATALINES;  
00134168           1

...
The values in the data are the numbers 1 and 2. The INPUT statement defined TYPE to be numeric. The expression in the IF statements use numeric constants.
Yes, this will work. As long as you are consistent you will not get any warnings or error messages and the Proc Print will appear as before.