
The laws surrounding citizenship by acquisition at birth abroad are very complex and specific. There are several sections of the law (INA) that may apply to an individual's specific facts.
Based on date of birth, there are different rules for how long the U.S. citizen needed to have been physically present in the U.S., including how long after certain ages. If inapplicable to your case, there could be other ways to prove citizenship, including in some instances using grandparent physical presence in the U.S.
Citizenship issues are far too complex and fact specific to really obtain accurate info through a Q&A. You need to consult with an experienced immigration attorney who can review your family's complete history and advise whether any citizenship options apply to you.
Regards,
Andrew M. Wilson, Esq.
Serotte Reich Wilson, LLP