How do I renew my US passport which expired 12 years ago?
You cannot renew by mail since you only qualify for that if your passport was issued in the last 15 years, and adult US passports are usually issued for 10 years (child passports are issued for 5 years), so your passport must have been issued 1722 years ago. Therefore, you have to apply for a new passport in person. Your expired passport, no matter how long it has been expired, serves as a proof of US citizenship for when you apply for a new passport.