"Nowadays, many children grow up in bilingual families and environments and thus acquire two languages as their first ones."
I will have to say that scientific research on this is somewhat scarce, but I myself have done lots of self-exploration, deep-diving and research, so I will provide you an extensive answer (from what I have acquired).
The subconscious, or unconscious mind as some scientists call it, is what we are only born with. There is no separation and we haven't developed the conscious mind yet. We are an open, blank-pages book ready to learn, absorb and experience life on planet Earth. From that point of view, we acquire the knowledge of a new language with the unconscious mind, which means, resistant-free. As we grow and socialize, in this age, sometimes we limit ourselves and restrict our flow by incorporating lot of intellectual reasoning behind our, and/or other people's behaviour, or whatever kind of phenomena we are trying and willing to give an intellectual background and meaning to. Therefore, sometimes our brains lose plasticity and our flow gets restricted, as I said. We restrict the growth of our other senses too, usually for the benefit of intellectual dominance and/or superiority. Children/babies do not have this because they/we are pure subconscious (depends on the age though of course), guided by the subconscious and hence, resistance-free.
But it's very possible for an adult to (re)learn to (recurrently) purify his/her body, mind and heart and bring it to a state of ecstasy and openness to receive whatever the conscious mind wants to receive. There are many techniques used to purify the subconscious and maintain its plasticity. So many people that have maintained their "elastic" brains/minds and speak multiple languages, have done so because they have full mastership of the desires of the subconscious and just let the conscious mind be the tool through which they set boundaries and let learn/do what they want to.
The conclusion is that even the ones here speaking multiple languages have succeeded in that because of using the conscious mind to start learning, and let the conscious AND subconscious do its job (which ultimately there is no escape from), which usually means using/making no real effort in your learning process, no effort at least that is not Your personal choice and desire. Subconscious is very easily programmed. With love, you can make it learn whatever your conscious mind/you, would like to learn, because you are one with it.
If this blog was or sounded too esoteric or complex to you, I apologize and am open to whatever questions and discussion.