The reason adults do not “absorb” language by exposure alone is because of something that psycholinguists refer to as the “critical period” of language development. Although it varies from person to person, adolescence is a good approximate cutoff, in that from as young as ten, and as late as the mid-teens, we lose our ability to acquire language without instruction the way children do. This is related to plasticity of the brain as it develops. Remarkably, although the spectrum of the critical period ends around adolescence, monolingual children generally complete the majority of the acquisition process by the age of three! There is always more vocabulary to acquire throughout childhood and life, but syntax (the order in which our words appear automatically when we speak, and in which they must do so in order to make sense to others) and other basic elements of linguistic grammar like phonology (sound patterns) and morphology (word components) are produced with ease before the end of preschool and toilet training! This is especially fast when you consider how complex and intricate human languages are. When children are exposed to a second language at four or five, they may still acquire it fluently if they get enough exposure to it, because that critical period of language development is still open.
Adults have a very different experience. Most adults can learn a second language, but it requires great effort, work, time, energy, and memorization. Plus, some elements, like vocabulary or native-like pronunciation, may never be fully achieved by adult language-learners (adult acquisition is more like learning, so linguists may use that verb more to describe second language acquisition). For example, many English speakers learning Spanish have trouble rolling the double r, and Hindi has a d sound with a puff of air that many English speakers cannot even hear, let alone pronounce (click on this UCLA link to see if you can!). You may speak another language at a rudimentary level, but never encounter thousands of low-frequency words like appendix or anesthesia, and hopefully, you’ll never have an urgent need to know those words while traveling.
A fascinating side note about the fact that all children universally acquire the language spoken to them at the same rate is that there is no such thing as a “simple” or “complicated” or an “easy-” or “difficult-to-learn” language. Some languages have more complex syntax, but very simple phonology. Others may have complex phonology, with many different sounds, but a smaller vocabulary, so there are slightly fewer words to learn.
What Does It Mean to Be Bilingual?
People who speak two or more languages fall on a spectrum of linguistic knowledge. One of the most common misconceptions is that bilingual adults are “balanced”—equally fluent in both—but this is rarely the case. A lot of adults who learned a second language later in life, and speak it very, very well, still recognize that they have an accent in the second language, or that they may not know every idiom or word like a native speaker does. They also may describe always speaking their first language fluently, no matter how many years go by. Conversely, some adults report that speaking that second language for many years can subtly but adversely affect their first language; perhaps when they travel to their home countries, they discover that they have lost some words or newer cultural references, and that they even pronounce that first language a bit stiffly, although this accent may disappear after a few days of being with family and native-speaker friends. We often hear language learners make mistakes that sound like their own first language, but believe it or not, second languages can influence our first language, too.
Sometimes people may exaggerate their language abilities, and claim to have acquired a second language in adulthood with fluency, when in fact they are only able to communicate in limited circumstances, such as when asking for directions or ordering food. This is still a form of bilingualism, but not as balanced as many imagine. Some scholars call them “situational bilinguals.”