Seeing how the vast majority of adults does in fact speak two languages and a huge chunk of them even more (e.g. most of Africa) I'd say your initial premise (adults struggle to learn anything past one) is misguided.
I assume that most bi-lingual or multi-lingual adults today picked up the fundamentals of all those languages as children. They didn't learn them after growing up.
Yet still after 40 years they are not as comfortable as with their original language.
>Also, your rebuttal does not consider the fraction of those adults who learned those languages when they were young.
That still includes:
-Pretty much all immigrants
-All populations using a regional lingua franca (Modern Standard Arabic, Swahili, Indonesian, French, English, Russian, Portuguese)
-Everyone who learned English as a requirement for their job (that's everyone in business, research, the tourism industry, the software industry and probably a few others I'm missing)
-Everyone who learned another language as part of their higher education (usually English, but can also include languages like French, German or Russian)
Of course the language acquisition is usually far from perfect. You probably won't get rid of your accent as an adult, and your writing may be rife with mistakes. But as a communication tool, most adults manage just fine, because communication is part of being human and humans are naturally good at communicating.