PL/SQL - Member Function: Link directly to object without parameters
Forget my comment.  I was thinking of something else.

Your question is really about "How do I access the MEMBER attributes from within a MEMBER function?".

In this case, you use the keyword SELF as the current instance's name.

Spec

CREATE TYPE "CUSTOMER" AS OBJECT (
    customer_id           NUMBER,
    customer_firstname    VARCHAR2(30),
    customer_lastname     VARCHAR2(30),
    customer_location_id  NUMBER,
    creation_date         DATE,
    MEMBER FUNCTION "FULL_YEARS" RETURN NUMBER
);
/


Body

CREATE OR REPLACE
TYPE BODY CUSTOMER AS

  MEMBER FUNCTION "FULL_YEARS" RETURN NUMBER
  as
     v_years  NUMBER;
  BEGIN
    -- don't do the SELECT INTO FROM DUAL thing.  that is just ugly code.
    v_years := FLOOR(MONTHS_BETWEEN( sysdate, self.creation_date)/12);

    RETURN v_years;
  END "FULL_YEARS";
END;
/