Douglass's best guess is that he was born in or around 1818, based on his master telling him in "some time during 1835, [that he] was about seventeen years old." He "was probably between seven and eight years old when [he] left Colonel Lloyd's plantation" to "live with Mr. Hugh Auld, brother to [his] old master's son-in-law." In chapter VI, he writes, "Very soon after I went to live with Mr. and Mrs. Auld, she very kindly commenced to teach me the A, B, C." This would make him around eight years old* when he began to learn to read (his learning to write came later).
* Online bios of Douglass routinely say he learned around 10, but this figure doesn't mesh with Douglass's narrative.
This would mean that he learned to read around 1826. The place was "Alliciana Street, near Mr. Gardner's ship-yard, on Fells Point" (Baltimore, Maryland).