I’m not sure what the sample size is, or how old the adults in the study are, but:

Ferman and Avi Karni from the University of Haifa, Israel, devised an experiment in which 8-year-olds, 12-year-olds and adults were given the chance to learn a new language rule. In the made-up rule, verbs were spelled and pronounced differently depending on whether they referred to an animate or inanimate object.

Participants were not told this, but were asked to listen to a list of correct noun-verb pairs, and then voice the correct verb given further nouns. The researchers had already established that 5-year-olds performed poorly at the task, and so did not include them in the study. All participants were tested again two months later to see what they remembered.

“The adults were consistently better in everything we measured,” says Ferman. When asked to apply the rule to new words, the 8-year-olds performed no better than chance, while most 12-year-olds and adults scored over 90 per cent. Adults fared best, and have great potential for learning new languages implicitly, says Ferman. Unlike the younger children, most adults and 12-year-olds worked out the way the rule worked – and once they did, their scores soared. This shows that explicit learning is also crucial, says Ferman, who presented the results at the International Congress for the Study of Child Language in Montreal, Canada, this week.

New Scientist: Age no excuse for failing to learn a new language