> all children seem to learn their respective mother tongues at about the same rate. I need a reference for this. We don't even learn different features or sou

> all children seem to learn their respective mother tongues at about the same rate. I need a reference for this. We don't even learn different features or sounds of our own languages at the same rate; some are harder and take longer, some are easier and we get them quickly. If this weren't true, there wouldn't be stereotyped ways that we have for imitating childish speech. This honestly sounds like the law of averages rearing its head again. There aren't many languages that five or ten years of immersion and few other responsibilities (like a child has) couldn't make easy, but that doesn't mean that all languages are equally difficult for their native speakers.