Toddlers are not inclined to perform tasks that are hard. So, they don't really make a choice about learning a language or not, neither is it based on necessity: before and after language acquisition, a baby would receive attention and protection. Before anyone says anything about reinforcement, kids with delayed language development (up to around 7) can be mute for years regardless of any coaxing. This is a special case though -- the brain seems to delay its pruning, almost as if to "wait" for the speech centers to kick into gear when speech begins.
Toddlers are innately imitative. Speech acquisition can be viewed as an imitative behavior, akin to, say, reaching out and grabbing something, kicking a ball, what not. The remarkable thing is that by observing something happening, they know how to control the analogous functions in their own bodies. I mention this as an alternative to this "necessity to communicate" idea: it could be largely the result of imitation, with the ability to communicate as a byproduct.
Finally, early languages take precedence over later languages. It could be because the early development of concrete operations, and later, abstract thought, rely on the available (i.e. first/native) language to "bootstrap" more complex thought processes. So the first language gets used non-stop. Then the second language comes in. Let's say you study it as an adult, and work real hard on it. When we fire a neural signal though, the wirings for the first language are so much stronger that they enjoy a lopsided privilege in access speed. In this sense, the later languages will never quite equal the earlier ones, by far.
So while I'm not saying anything against the adult ability of new language acquisition (and by this I mean a different set of phonemes and grammar -- in-family languages like Italian to Spanish won't count), I would say there is something quite magical about the child learning their first language(s).
If you learned one separately from another, after the first was stable already, the new language will occupy an adjacent region (somehow I think this is because the region is still growing).
All languages will be functionally native (assuming early enough acquisition and abundant practice for all), but in the latter case, you would still expect a slightly weaker signal for the later languages. But in both cases, I would just count all native-level languages as first languages, taking "minimal accent" as the litmus test (some people slip through, and in my experience those people had amazing ears and were gifted with languages in general).
This is just anecdote, but from all my friends who have began learning English at a different age, their accents seem to follow a general pattern of later acquisition -> stronger accent. While this seems obvious, for those who learned English around 12-14, you get an interesting effect of native-level fluency, but just at certain places, you can tell it isn't their first language. And usually, at least for my friends, you can tell about what age they started learning.
mirror system hypothesis
http://www.isrl.uiuc.edu/~amag/langev/paper/arbib06mirrorSys...
My dad spoke and sang to me a little bit in Chinese when I was a toddler, but it never stuck. I suspect this is because I could always revert to English and he'd talk back to me in English, so there was no reason for me to learn Chinese. Some of my cousins - the ones whose parents were more strict about speaking Chinese at home - did end up picking it up.
Similarly, this also explains why immersion works, why immigrant ghettoes prevent language acquisition, and why people retain accents. If you're dropped in a foreign culture and have to learn the language to survive, you'll manage it whether you're 10 or 60. If you have other people who speak your language that you can depend on, there's much less reason to learn a foreign language. And if you can speak well enough to be understand (as most people with accents can), there's no reason to improve that last 10% (which, in true 90-10 form, takes 90% of the effort) to make yourself sound like a native speaker.