The Mary Prentiss Inn near Porter Square. (Photo: bagaball via Flickr) The 20-room Mary Prentiss Inn near Porter Square has sold for $6.1 million after more than a year on the market. The bed and breakfast occupies a two-story, 13,549-square-foot structure built in 1840 at 6 Prentiss St. in the Baldwin neighborhood. It was sold…