Currently there is no way to change your Date of Birth, either for fixing mistakes or other reasons.
I'm not sure how FN's backend works, but it shouldn't be more then a few changes in the database.
Also if this is to prevent minors from going above "18 year" to allow them to see "mature content", its a lame way to do it. Its not like its actually preventing them from seeing mature content, Where there is a will there is a way.
Right now its only annoying.
Customer support service by UserEcho