printing variables in pl/sql

You can use below to print Boolean Value in PLSQL

dbms_output.put_line('v_six_years '||  sys.diutil.bool_to_int(v_six_years));

It seems you cannot concat varchar and boolean.

Define this function:

FUNCTION BOOLEAN_TO_CHAR(FLAG IN BOOLEAN)
RETURN VARCHAR2 IS
BEGIN
  RETURN
   CASE FLAG
     WHEN TRUE THEN 'TRUE'
     WHEN FALSE THEN 'FALSE'
     ELSE 'NULL'
   END;
END;

and use it like this:

DBMS_OUTPUT.PUT_LINE('flag '|| BOOLEAN_TO_CHAR(v_six_years));

PL/SQL does not have a literal to represent boolean values. You will have to either convert the v_six_years boolean value to a string, or not use a boolean if you wish to print the value. PL/SQL booleans are great for logic but useless if you wish to display the value.

DECLARE
   v_hire_date DATE:='30-Oct-2000';
   v_six_years VARCHAR2(1);  
BEGIN
IF MONTHS_BETWEEN(SYSDATE,v_fecha_contrato)/12 > 6 THEN
      v_six_years:='1';
ELSE
      v_six_years:='0';
END IF;
DBMS_OUTPUT.PUT_LINE('flag '||v_six_years);
END;

Or

DECLARE
   v_hire_date DATE:='30-Oct-2000';
   v_six_years BOOLEAN;  
   v_six_years_display VARCHAR2(5);
BEGIN
IF MONTHS_BETWEEN(SYSDATE,v_fecha_contrato)/12 > 6 THEN
      v_six_years:=TRUE;
      v_six_years_display := 'true';
ELSE
      v_six_years:=FALSE;
      v_six_years_display := 'false';
END IF;
DBMS_OUTPUT.PUT_LINE('flag '||v_six_years_display);
END;

dbms_output.put_line is not overloaded to accept a boolean argument.Simple one line answer would be

dbms_output.put_line(case when v_six_years = true then 'true' else 'false' end );