Hi all,

I have a dataset with the launch_date (data_lancio) for each molecule, which is repeated over and over the sample. What I would like to do is to create a variable called "age_molecule" and to do that I need to see which is the first occurrence of the variable molecule with respect to the launch date. In other words, my dataset is made like this:

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input int data_lancio str60 molecule
1924 "ALOE BARBADENSIS"                 
1929 "PURGATIVE/LAXATIVE"               
1929 "ACETYLSALICYLIC ACID"             
1929 "DEXTROMETHORPHAN"                 
1929 "HYDROGEN PEROXIDE UREA DERIVATIVE"
1929 "DIPHENHYDRAMINE"                  
1929 "DIPHENHYDRAMINE"                  
1929 "MECLOZINE"                        
1929 "INULIN"                           
1929 "OXYMETAZOLINE"                    
1929 "DEXTROMETHORPHAN"                 
1929 "UREA"                             
1929 "PARACETAMOL"                      
1942 "INFANT MILKS"                     
1942 "TOLTERODINE"                      
1942 "METHYLPHENIDATE"                  
1942 "EPLERENONE"                       
1942 "INFANT MILKS"                     
1942 "POLYCARBOPHIL"                    
1942 "INFANT MILKS"                     
1942 "PHENOBARBITAL"                    
1942 "IRON FERROUS"                     
1942 "ASCORBIC ACID"                    
1942 "BENZOCAINE"                       
1942 "INFANT MILKS"                     
1942 "FEEDING BOTTLES"                  
1942 "MORPHINE"                         
1942 "NUTRITIONAL SUPPLEMENTS"          
1942 "INFANT MILKS"                     
1942 "CALCIUM"                          
1942 "CALCIUM"                          
1942 "DEXTROMETHORPHAN"                 
1942 "KETOPROFEN"                       
1942 "NUTRITIONAL SUPPLEMENTS"          
1942 "INFANT MILKS"                     
1942 "INFANT MILKS"                     
1942 "INFANT MILKS"                     
1949 "INFANT MILKS"                     
1949 "IRINOTECAN"                       
1949 "GLIPIZIDE"                        
1949 "DICLOFENAC"                       
1949 "CLOBETASOL"                       
1949 "ASCORBIC ACID"                    
1949 "FLUOCINONIDE"                     
1949 "FERUMOXIDES"                      
1949 "PSEUDOEPHEDRINE"                  
1949 "AMANTADINE"                       
1949 "DISPOSABLE MEDICAL DEVICES"       
1949 "ERGOCALCIFEROL"                   
1949 "NARATRIPTAN"                      
1949 "BETAXOLOL"                        
1949 "FOLIC ACID"                       
1949 "COMPOSITION UNKNOWN"              
1949 "PSEUDOEPHEDRINE"                  
1949 "SALICYLIC ACID"                   
1949 "NAPROXEN"                         
1949 "GLUCOSE, URINE TESTS"             
1949 "PROTEIN TESTS"                    
1949 "GENERAL NUTRIENTS"                
1949 "ESTRADIOL"                        
1949 "TESTOSTERONE"                     
1949 "PROCHLORPERAZINE"                 
1949 "COMPOSITION UNKNOWN"              
1949 "NUTRITIONAL SUPPLEMENTS"          
1949 "APRACLONIDINE"                    
1953 "ACETAZOLAMIDE"                    
1953 "NYSTATIN"                         
1953 "MONTELUKAST"                      
1953 "GABAPENTIN"                       
1953 "NOREPINEPHRINE"                   
1953 "NAPROXEN"                         
1953 "NYSTATIN"                         
1953 "PSEUDOEPHEDRINE"                  
1953 "ROCURONIUM BROMIDE"               
1953 "COMPOSITION UNKNOWN"              
1953 "VORICONAZOLE"                     
1953 "ETOPOSIDE"                        
1954 "ALFUZOSIN"                        
1954 "NON-DISPOSABLE MEDICAL DEVICE"    
1954 "BUSPIRONE"                        
1954 "DIGOXIN"                          
1954 "METFORMIN"                        
1954 "ATROPINE"                         
1954 "PARAFFIN OIL"                     
1954 "BUSPIRONE"                        
1954 "ZONISAMIDE"                       
1954 "ZONISAMIDE"                       
1954 "ATROPINE"                         
1954 "8-QUINOLINOL"                     
1954 "PRAVASTATIN"                      
1954 "OTHER CLEANSING AGENTS"           
1954 "BELATACEPT"                       
1954 "TENIPOSIDE"                       
1954 "MISCELLANEOUS URINE TESTS"        
1954 "CLONIDINE"                        
1954 "PREDNISOLONE"                     
1954 "ATROPINE"                         
1954 "CLONIDINE"                        
1954 "ALFUZOSIN"                        
1954 "ATROPINE"                         
end
where molecule is not unique for each launch date. I would like to take the minimum launch date for each molecule. So for instance, CALCIUM here appears just for 1942 but in the complete sample it also appears for 2004, 2005, 2012 and 2015. I would like to take the 1942 as launch date for CALCIUM and do the same for all the other molecules.

Many thanks!