A 75-year-old man and a 35-year-old woman were arrested on prostitution-related charges in Roxbury Monday. Boston police said Nathan Andrews of Roxbury, who also goes by the name Fard Ahmed, was charging $20 for a two-hour room rental and selling condoms and alcohol at 131 Eustis St. He was charged with allowing a premise to be used for immoral purposes. Shannon-Lynn Quigley of Jamaica Plain was arrested on a charge of engaging in sex for a fee, police said. Another property owned by Andrews, which was suspected of being used for prostitution, was shut down by the city in August 2011 for code violations.