Inspecting the LOG
The SAS log reports some of what is going on with DOB.

2    DATA WontWork;  
3       INPUT ID LASTNAME $ DOB HEIGHT WEIGHT GENDER $ AGE;  
4       FORMAT DOB MMDDYY8.;  
5    DATALINES;
  
NOTE: Invalid data for DOB in line 6 9-15.  
RULE:----+----1----+----2----+----3----+----4----+----5----+----6  
6    1 SMITH 1/23/66 68 144 M 26  
ID=1 LASTNAME=SMITH DOB=. HEIGHT=68 WEIGHT=144 GENDER=M AGE=26 _ERROR_=1 _N_=1  
NOTE: Invalid data for DOB in line 7 9-15.  
7    2 JONES 3/14/60 78 202 M 32  
ID=2 LASTNAME=JONES DOB=. HEIGHT=78 WEIGHT=202 GENDER=M AGE=32 _ERROR_=1 _N_=2  
NOTE: Invalid data for DOB in line 8 7-14.  
8    3 DOE 11/26/47 62 99 F 45  
ID=3 LASTNAME=DOE DOB=. HEIGHT=62 WEIGHT=99 GENDER=F AGE=45 _ERROR_=1 _N_=3  
NOTE: Invalid data for DOB in line 9 14-19.  
9    4 WASHINGTON 8/1/70 66 101 F 22  
ID=4 LASTNAME=WASHINGT DOB=. HEIGHT=66 WEIGHT=101 GENDER=F AGE=22 _ERROR_=1 _N_=4  
NOTE: The data set WORK.WONTWORK has 4 observations and 7 variables.