Quick answer
An age declaration is a sworn statement about a person’s date of birth, usually from a court process. Nigerian passport guidance mentions NPC birth certificate or declaration of age in some application contexts.
Birth certificate vs age declaration
An NPC birth certificate is an official civil registration record. An age declaration is typically a sworn statement used where birth registration was not available or was not done at the time.
Before using it
Ask the institution requesting the document whether they accept age declaration, NPC birth certificate, birth attestation, or another document. Different agencies and embassies may apply different rules.
Related: Birth certificate Nigeria, Passport renewal.