Children learn mainly by focusing subconsciously on the content of words, while adults struggle with the form. Adults' efforts are mainly focused on lexical and grammatical accuracy, while children concentrate on the message they want to get across. Having their abstract thinking ability more developed, adults don't need to appeal to their senses to understand concepts. Children internalize concepts by doing physical activities and hands-on practices. They rely heavily on all their senses and are able to remember many things because of the connection they make between their actions and the language they learn. All this means is that younger people often find it easier to learn foreign languages than their adult counterparts.
Introverted people may find it difficult to learn to communicate in a foreign language, because mastering a language means, first of all, being able to speak it. More communicative students aren't afraid to take risks or make mistakes as long as the listeners understand the meaning of their messages. Shy students try to avoid speaking in a foreign language because it sounds weird to them. Thus, the acquisition of the language is much slower because they get little practice.
Foreign language learning is also affected by motivation. If learners like learning or speaking the language (intrinsic motivation), they're able to make sustained efforts to acquire it. They can also be motivated to learn a foreign language if they have a specific purpose, such as to pass a school exam or get a promotion (extrinsic motivation). The expectation of a reward of some sort is always a strong motivator and it plays an important part in the acquisition of a foreign language.
The process of learning a foreign language can differ in time length and/or proficiency if students are in a submersion context. Here, they attend regular content-area classes with no previous foreign language instruction, thus "absorbing" the language while focusing on the subject matter. Most students learn faster in a submersion context because they're "pushed" to learn, but some have difficulties in both the acquisition of the language and the subject matter. In this situation, students attend classes in foreign language instruction for just a period or so every day.
In immersion contexts, students attend specially designed courses. They all have the same native language and the same proficiency level in the target language. The acquisition process is slower, but more reliable in the long run. Even if students can communicate in their native language, this possibility is only a security blanket to improve self-confidence and risk taking. In immersion contexts, students learn the language in context, at their pace and at their level.