One of the requirements of an affidavit is the statement
that the person who sign the affidavit must have have personal knowledge of your birth. Mostly, the statement given indicates that the person was present when you were born.
Legal age means that they are older to you in a way that they would be "legal" adults when you wre born. As an example, your uncle was 20 years old when you were born, and he can give an affidavit that he was present at the time of your birth.
However if your cousin, say is 3 years older to you, he cannot sign an affidavit that he was present at birth, because even if he was, he was not of legal age at that time.