This is simply false. E.g., my kid is around speakers of 4 rather different languages. At age 3, he has grasped all 4 languages to a good degree and even transl

This is simply false. E.g., my kid is around speakers of 4 rather different languages. At age 3, he has grasped all 4 languages to a good degree and even translates pretty well between them. I don't think he's particularly unusual and 3-4 language speaking kids are reasonably common around here.