If because of a physical or mental condition you did not apply for a period of disability within 12 months after your period of disability ended, you may apply not more than 36 months after the month in which your disability ended. Your failure to apply within the 12-month time period will be considered due to a physical or mental condition if during this time—

(a) Your physical condition limited your activities to such an extent that you could not complete and sign an application; or

(b) You were mentally incompetent.