@Lustig Andrei,
David's notion is preposterous, Merry, but it's a common enough preposterousness.
The distinction I had in mind is the manner of 'learn'. People still hold to the notion that we, adults, teach mother tongue/milk language children their language.
For children, it doesn't matter what the first language is, what language grouping it comes from, they learn a 2nd, 3rd, ... language pretty much like their first if they are immersed in the new language. There certainly is a "critical" period but the manner in which children 'learn' language is the distinctive nuance of 'learn' that I was thinking of.