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Adult slavery was abolished in Vermont in July 1777 by a provision in that state's Constitution that male slaves become free at the age of 21 and females at the age of 18.  However, violations of the law against adult slavery were not unusual.
Chapter I of the Constitution, titled "A Declaration of the Rights of the Inhabitants of the State of Vermont" said:
... no male person, born in this country, or brought from over sea, ought to be holden by law, to serve any person, as a servant, slave or apprentice, after he arrives to the age of twenty-one Years, nor female, in like manner, after she arrives to the age of eighteen years, unless they are bound by their own consent, after they arrive to such age, or bound by law, for the payment of debts, damages, fines, costs, or the like. 
The state of Vermont was created in 1777 by politicians in Vermont defying New York, which then claimed Vermont was legally a part of New York, and creating a popular government that represented their interests, among them abolishing slavery. After 1777, Vermont was repeatedly denied admission to the Union and existed as a largely unrecognized state until it was admitted to the Union as the fourteenth state in 1791. Vermont's admission to the Union made the state subject to the Fugitive Slave Clause of the Constitution of the United States (Article IV, Section 2, Clause 3) requiring fugitive slaves fleeing into a state whose laws forbid slavery to be returned. Later the state was subject to the Fugitive Slave Acts of 1793 and 1850, allowing slave owners to recover fugitive slaves who fled to Vermont.
Harvey Amani Whitfield's book, The Problem of Slavery in Early Vermont, reports that among those violating the abolition of slavery were Vermont Supreme Court Judge Stephen Jacob and Levi Allen, brother of the military leader Ethan Allen. 
The 1790 census counted 16 slaves in Vermont, all in Bennington County. In 1870, the chief clerk of the Census Bureau, who was from Vermont, changed the reported status of the 16 to "Free Other", alleging that the original report was a mistake. Whether it was really a mistake is a matter of some dispute.  (The 1790 census of the United States did not reach Vermont until the following year, 1791, because the government of Vermont took the position that Vermont was not a part of the United States until its admission to the Union in 1791.)