The research on dual language learners, whether GT or NT, is that their early language milestones tend to be somewhat delayed, but that by school-age, they have caught up, and are beginning to exceed the metalinguistic skills of monolingual speakers. If they maintain and continue to develop their multilingual skills, their long-term prospects for metalinguistics are quite strong, as (somewhat surprisingly) are their executive functions. There have been some data that suggest that the need to inhibit the other languages contributes to better overall inhibitory control (impulse control).
