The simple answer to that question would be yes, of course they can! However, it’s slightly more difficult for an adult to learn a second language than it is for a child, and that’s because of the way the human brain works.
Linguists have suggested that humans have a ‘Critical Period’ in the early stages of their life, in which they are more open to environmental stimuli. In terms of language, the critical period spans from birth to around seven years old: in this phase of life, the brain is more easily moulded, and that is why children learn so fast. Because of this, children can learn two languages, from birth and never really confuse the two.
Past the age of seven, however, humans lose the ability to learn in the same way. Adults can still learn a new language, it just takes a little more work. There are many theories about the way children learn their first language, and we can actually apply these theories and techniques to ourselves when learning a second language.
First is the behaviourist approach. This idea states that children acquire new language by copying the adults around them. In the case of the adult learner, it is important to be around native speakers of the language, and to be in contact with native forms of the language. Let’s take Spanish, for example, watching TV shows in your target language or finding an online chat forum with native speakers will help you to progress and improve your Spanish vocabulary, as you can listen to how the language and the Spanish words are used and imitate certain sentence formations or phrases –particularly useful when learning idioms and metaphors. Language learning via immersion (i.e. moving to a country that speaks it) is one of the quickest ways to learn a language, albeit one of the scariest.
Similarly, other linguists have suggested that children need contact with a “more knowledgeable other,” i.e. somebody who is above the level of speaking you are at who can help to push you into the next phase of learning. A teacher is best here, but any contact with native speakers is good practice.
Studies have shown that grammatical development is something that takes place through usage of the language and making mistakes along the way. For children, grammar is an extremely hard concept to grasp, and for this reason, science has proven that adults can out-perform children at learning language when it comes to this area. Short-term, adults can pick up grammar much quicker, because we have a basic understanding of it in our own language, and can adapt that knowledge to apply to something new. So, adults aren’t completely disadvantaged when it comes to learning a second language! In fact, the only area where adults are more hindered is pronunciation, because certain languages have sounds that we don’t possess in our own.
To fully answer the title question, adults can most definitely become fluently bilingual. All it takes is a willingness to put in effort, and enough nerve to talk to other speakers. Be confident! We might not have a child-like neuroplasticity, but don’t let that put you off. It’s never too late to start learning.