As someone who studied English as an A subject at university, I’d say that 5–6 is optimal for learning the second language. They taught us this is a good age because the phonetic system is then fully developed and the child has enough cognitive capacity for this process.

Of course, there are children who are taught 2 languages since birth, even 3, but that usually confuses them. Also, it takes them even more time to speak each language properly compared to monolingual children. Even if you teach your child 2 languages early, it doesn’t necessarily make them bilingual later in life.

But those who sell language courses can prove me wrong.