Because he was born in slavery, Frederick Douglass has no accurate knowledge of his birthdate, "never having seen any authentic record containing it".  Douglass explains that most masters sought to keep their slaves in ignorance concerning their birthdates, saying,

"By far the larger part of the slaves know as little of their ages as horses know of theirs, and it is the wish of most masters within my knowledge to keep their slaves thus ignorant...I was not allowed to make any inquiries of my master concerning it.  He deemed all such inquiries on the part of a slave improper and impertinent, and evidence of a restless spirit" (Chapter 1).

By denying slaves even this basic knowledge, masters hoped to prevent them from developing a sense of their individuality and humanity, because they feared that this woud lead them to question their station in life and become rightfully rebellious.