This information isn't specific to those over age 18, but perhaps will still be helpful to you. The US Census Bureau published a report based on 2012 information, English-Speaking Ability of the Foreign-Born Population in the United States: 2012. According to that report, 44 percent of the foreign-born population age 5 and older who came to the United States in 2000 or later said that they had high speaking ability in English, saying they spoke English very well, some indicating that they spoke only English at home. Only 13% said they did not speak English at all.