SET SERVEROUTPUT ON;
DECLARE
  v_num  NUMBER;
  v_days NUMBER;
BEGIN
  SELECT to_date(sysdate)-to_date(dob)
  INTO v_num
  FROM customer_details
  WHERE application_id=103;
  v_days             :=floor(v_num/366);
  dbms_output.put_line(v_days);
END;
/

DECLARE
  v_num  NUMBER;
  v_days NUMBER;
BEGIN
  SELECT to_date(sysdate)-to_date(dob)
  INTO v_num
  FROM customer_details
  WHERE application_id=:block3.day;
  v_days             :=floor(v_num/366);
  MESSAGE(v_days);
  MESSAGE(v_days);
END;

to_date(sysdate)

to_date(to_char(sysdate, <nls_date_format>), nls_date_format)

SELECT to_date(TO_CHAR(sysdate,'DD/MM/YYYY'),'DD/MM/YYYY')-
  to_date(TO_CHAR(dob,'DD/MM/YYYY'),'DD/MM/YYYY')
  INTO v_sub
  FROM customer_details
  WHERE application_id=p_id;
  v_dobcalc          :=ROUND(v_sub/366);

select sysdate - dob
  into customer_details
  from ...