Of foreign-born Hispanics, those who have become naturalized citizens are more likely to speak English very well or pretty well than those who are not citizens (52% versus 25%). At the other extreme, nearly three-quarters (73%) of non-citizens say they speak just a little or no English compared with 46% of naturalized citizens.
Foreign born Hispanics are more likely to speak English if they have lived in the country longer and have more education. Education also accounts for much of the different in English proficiency between foreign born Hispanics from different countries. Age of immigration is also critical:
Adult immigrants who arrived as young children are more skilled at English than are those who crossed the border when they were older. Three-quarters (76%) of foreign-born Latino adults who arrived at ages 10 or younger report that they can carry on a conversation in English very well. That compares with 30% of those who arrived at ages 11 to 17; 16% of those who arrived at ages 18 to 25; and 11% of those who arrived at ages 26 or older.
Add those who speak English "pretty well" and you are approaching 80% English proficiency for those who immigrate at ages 10 or younger.
Source: Meta-analysis by the Pew Hispanic Center.
None of this is surprising. Indeed, it is entirely consistent with both the literature on immigrant language acquisition, family discussions of language acquisition by my immigrant ancestors, and my own personal experiences in multiple immigrant communities.
Indeed, the research shows that if you spent your infancy abroad in a place where a foreign language was spoken, then come to the United States, that you will lose your ability to speak your language of birth if it is not maintained. Language acquisition is basically hard wired. People automatically and almost effortlessly learn the languages spoken around them as children. The ability then wanes, eventually ending when we become adults.
The myth that Hispanics will assimilate into the United States less than prior waves of immigrants have is simply untrue. Foreign born adults are never going to have perfect English accents. But, the vast majority of foreign born persons who come to the United States as young children, and almost everyone born in the United States is linguistically and culturally an American.
If we want citizenship requirements to match some sort of defacto reality of assimilation into United States culture (a normative question which I don't necessarily agree with, but which is implicit in much of our naturalization law), then we should not only retain birthright citizenship, but should further extend it to anyone who has lived in the United States to adulthood or near adulthood, from the time they were young -- a cutoff that is no younger than age ten, and probably several years older. The scholarly literature (e.g. this study)shows that late youth shows a partial decline in average language acquisition ability which plateaus at a lower average level for adults.