Answer by taking a quote from the following article:

Frederick Douglass (born Frederick Augustus Washington Bailey; c. February 1818 - February 20, 1895) was an African-American social reformer, abolitionist, orator, writer, and statesman. After escaping from slavery in Maryland, he became a national leader of the abolitionist movement in Massachusetts and New York, gaining note for his oratory and incisive antislavery writings. In his time, he was described by abolitionists as a living counter-example to slaveholders' arguments that slaves lacked the intellectual capacity to function as independent American citizens. Northerners at the time found it hard to believe that such a great orator had once been a slave.

The couple settled in New Bedford, Massachusetts. After meeting and staying with Nathan and Mary Johnson, they adopted Douglass as their married name: previously Douglass had been given the second name of Bailey, but he now again felt the need to find a new name and asked Johnson to choose a suitable surname. Johnson had been reading The Lady of the Lake, and suggested "Douglass" after the principal character in the poem.  Douglass thought of joining a white Methodist Church but from the beginning he was disappointed when he saw it was segregated. Later he joined the African Methodist Episcopal Zion Church, an independent black denomination first established in New York City, which counted among its members Sojourner Truth and Harriet Tubman. He became a licensed preacher in 1839, and this helped him hone his oratorical skills. He held various positions, including steward, Sunday School superintendent, and sexton. In 1840, Douglass delivered a speech in Elmira, New York, then a station on the Underground Railroad. (Years later a black congregation formed there and by 1940 it became the region's largest church).  Douglass also joined several organizations in New Bedford, and regularly attended abolitionist meetings. He subscribed to William Lloyd Garrison's weekly journal The Liberator. Inspired by Garrison, Douglass later said, "no face and form ever impressed me with such sentiments [of the hatred of slavery] as did those of William Lloyd Garrison." So deep was this influence that in his last biography, Douglass confessed "his paper took a place in my heart second only to The Bible." Garrison was likewise impressed with Douglass, and had written about his anti-colonialism stance in The Liberator as early as 1839. In 1841, Douglass first heard Garrison speak at a meeting of the Bristol Anti-Slavery Society. At another meeting, Douglass was unexpectedly invited to speak. After telling his story, Douglass was encouraged to become an anti-slavery lecturer. A few days later Douglass spoke at the Massachusetts Anti-Slavery Society's annual convention in Nantucket. Then 23 years old, Douglass conquered his nervousness and gave an eloquent speech about his rough life as a slave.  In 1843, Douglass joined other speakers in the American Anti-Slavery Society's "Hundred Conventions" project, a six-month tour at meeting halls throughout the Eastern and Midwestern United States. During this tour, slavery supporters frequently accosted Douglass. At a lecture in Pendleton, Indiana, an angry mob chased and beat Douglass before a local Quaker family, the Hardys, rescued him. His hand was broken in the attack; it healed improperly and bothered him for the rest of his life. A stone marker in Falls Park in the Pendleton Historic District commemorates this event.

when did he become an abolitionist?
In 1843,