premise: Frederick William Chesson (1833–1888) was an English journalist and prominent anti-slavery campaigner. He was active in the London Aborigines' Protection Society and Emancipation Committee, and met Harriet Ann Jacobs when she was in England in 1858; and was a vocal supporter of the Union side during the American Civil War.
hypothesis: The writer met Jacobs when he was in his twenties.
reason: Fredrick William Chesson was born in 1833 and met Harriet Ann Jacobs  in 1858 so he would have been in his 20's (approximately 25) when they met.