Please list the ID of the patient whose RF is normal and who is older than 60.
RF is normal refers to RF < 20; older than 60 = SUBTRACT((YEAR(CURDATE()), YEAR(Birthday))) > 60;
SELECT T1.ID FROM Patient AS T1 INNER JOIN Laboratory AS T2 ON T1.ID = T2.ID WHERE T2.RF < 20 AND STRFTIME('%Y', DATE('now')) - STRFTIME('%Y', T1.Birthday) > 60