Bilingualism research has shown that the ability to focus one’s attention in the face of distraction develops earlier in bilingual children than in their monolingual peers (e.g., Bialystok, 1999; Martin-Rhee & Bialystok, 2008), and that lifelong bilingualism may protect older individuals from some areas of the cognitive decline that comes with normal aging (e.g., Bialystok, Craik, Klein, & Viswanathan, 2004; Bialystok, Craik, & Ruocco, 2006). While it is plausible to suppose that exceptional cognitive control is a felicitous by-product of the constant practice bilinguals are said to have with focusing their attention on the use of one language while suppressing interference from the other, the nature of the relationship between language-specific skill and domain-general cognitive control remains to be clarified, particularly among middle-aged individuals who are at the age at which the effects of cognitive aging commonly begin to manifest. With this study we sought to elucidate the relationship between language-specific proficiencies and general cognitive control by comparing the performance of Spanish/English bilinguals of different ages and language proficiencies on a series of cognitively demanding tasks. Participants were two groups of highly proficient Spanish/English bilinguals aged in their 20s or 40s/50s, and two age-matched groups of less-highly proficient Spanish/English bilinguals. Proficiency in Spanish and English was evaluated by participants’ self-assessments and by scores on independent measures of language proficiency. Cognitive control was assessed via complex sentence processing tests and via the Simon task, which tests the ability to respond to one type of visual stimulus (color) while suppressing attention to a competing visual stimulus (spatial orientation). Analyses of variance and multiple regressions were conducted to (1) compare the cognitive functioning of younger and older bilinguals, and (2) determine the predictive power of chronological age, age of onset of bilingualism, and language proficiency for efficiency of cognitive functioning among younger and middle-aged bilinguals.