Learning languages simultaneously during development, ideally before age 12, is optimal for becoming bilingual or trilingual without an accent.

“Can we say that there is a most complicated language out there, or among the most complicated? Would it be Russian? - It's definitely high up there. English is up there, too, actually. Yeah, German as well. - And in terms of learning multiple languages during development, my understanding is that if one wants to become bilingual or trilingual, best to learn those languages simultaneously during development, ideally before age 12, if one hopes to not have an accent in speaking them later. Is that correct, or do you want to revise that?”