Preliminary results from a longitudinal study of English-speaking children and adults learning Dutch in natural situations suggest that 12- to 15-year-olds learned faster than either older or younger subjects during their first 6 months in Holland. All age differences had disappeared in a group of advanced subjects (English-speakers who had been in Holland for more than a year). The mistakes made by the second language learners on a translation task were very similar to the kinds of mistakes made by young children in their first language. It is suggested that the source of mistakes in both first and second language acquisition is a lack of isomorphism between knowledge of the world and the distinctions expressed in the language. In first and second language acquisition, this is caused by relatively arbitrary linguistic subsystems; in second language acquisition it can also be caused by interference from the distinctions learned in the first language. First and second language acquisition are very similar in the stages and strategies which characterize them, and there is no reason to believe that young children learn language more easily than or in a different way from older children and adults. (Author)