Furthermore, some linguists say, the question really is whether the differences between the ways an adult's brain and a child's brain are organized for language learning amount to an adult impairment or simply a different way of learning. Although some adults suffer from memory loss and other debilitating factors, to many linguists, the answer is more commonly the latter.
"You have to teach differently, depending on the age," said DeKeyser, who is joining the University of Maryland at College Park as a professor in the fall and is the incoming editor of the scientific journal "Language Learning."
A child's brain is better able to absorb new sounds. Children also learn better through informal teaching, and adults do better through formal lessons, DeKeyser said. But individual aptitude, methods of teaching and time on task play roles as well.
"It is utterly naive to say the problems are solved just because you start early," he said.
Individual aptitude is little understood by researchers but is important in second language acquisition -- in young people and adults.
Peg Willingham, senior director for public sector development for the International AIDS Vaccine Initiative, took the Modern Language Aptitude Test years ago when she entered the U.S. Foreign Service, and aced it.
She is sure, she said, that her aptitude is "clearly genetic," as she comes from a long line of linguists; her great-grandfather spoke eight languages. She can speak varying degrees of five languages, including Arabic. (She started learning Spanish in eighth grade, French in ninth, German and Russian in college and Arabic as an adult.)
Other people can study Spanish for years and find themselves unable to hold a simple conversation in the language.
"One simple way to think about critical period effects in language is in terms of sports and music," said Robert Kluender, associate professor and chairman of the linguistics department at the University of California at San Diego. "It's very clear that early exposure facilitates the acquisition of the motor and cognitive skills required for high-level performance. Yet we don't all end up as Tiger Woods or Michelle Kwan or the Williams sisters. There, individual variation plays just as big a role."
Another key issue in language learning is time on task. Though adults and children learn differently, both need plenty of time to learn a language, underscoring a key problem with many school programs. Language learning takes hundreds of hours to reach any level of proficiency.
But in many places, children might meet once or twice a week for no more than 45 minutes -- hardly enough time for the implicit learning process at which children excel to work. This helps explain why many people can take years of foreign language classes and wind up not knowing much.
"One week abroad gives more exposure than a whole semester in class in terms of listening comprehension, which I consider the skill least emphasized in many methods but perhaps the most important," said Ron Takalo, associate professor of Spanish at Northwestern College in Iowa.
Mayer once lived in Germany, in fact, and he picked up German by assimilation rather than by formal instruction.
Now, at Temple Baptist Church in Northwest Washington, as part of the institute that is affiliated with American University, he and 68-year-old Ann Peterson and Mintzes enjoy the joys of working with another language.
"People with expansive intellectual interests like to learn new things at any age," Mintzes said.
