Somewhat related, I found my Son's bilingual exposure very interesting. From 3 months to 20 months he was in a bi-lingual montessori (english & spanish). Teache

Somewhat related, I found my Son's bilingual exposure very interesting. From 3 months to 20 months he was in a bi-lingual montessori (english & spanish). Teachers and assistants always use both languages to work with children. We also reinforced at home wherever possible. Our limited spanish skills were mostly from the things he brought home and told us and a few books. Then we moved and he was in another school, english only, from 21 months to 38 months. The decline of spanish skills was slow at first, even while we worked with him at home, then became very rapid. English skills continually improved, as would be expected. At 39 months we moved back to the original bilingual montessori. He quickly learned/learns new spanish words and phrases, but the previous ones (colors, numbers, shapes, animals) are mostly gone. Even though I recall 90% of them, all learned as an adult, he doesn't.