This is a very interesting question! I also wonder if it is, much like many other skills and processes, acquired as opposed to learned within the critical period.

This is a complex, multi-faceted question. However to give a brief answer, acculturation between youth and their parents tend to occur differently. For example, children tend to assimilate much more easily and quickly to the destination culture whereas parents tend to strive to adhere to their heritage culture. Accordingly, children often are more likely to adopt the values of the mainstream society while parents adhere more strongly to the values and cultural practices of their heritage culture. Thus, children are more likely to be bicultural and obtain greater intercultural competence. Differences in values and culture that stem from parent-child descrepanices are often the source of family conflict.

Regarding your question about language acquisition, children tend to adopt and adhere more strongly to the language of the destination culture relative to their parents. According to a linguist I spoke to recently, this is because children generally spend more time having to use the new language due to school, television, music, and peers from the new country of settlement. Below is a link to a classical article that addresses these questinos more in depth.

https://openaccess.leidenuniv.nl/bitstream/handle/1887/16610/AppPsy2006.pdf?sequence=2

I agree with @corycobb. I would like to add that children or young people who are born (migrate ro travel to) another country assimilate both language and culture of that country faster then than their parents. They develop hybrid or transnational identity that is not confined to geographical borders. For more information, please, see Article Narrating the Self among Arab Americans: A Bridging Discours...