Every person has their first language, or native language, which is also known as L1. The language which is not the native language is the second language or L2. The person learns this language later in life. The second-language can be any foreign language; even another language of the home country of the speaker is also a second language. It is not necessary that the speaker’s dominant language is his first language.
You can get or learn a second language. Acquiring and learning are two different things. Acquisition of a second-language is a natural process. And learning is a conscious process. So, for many people, acquiring a second-language is a lifelong learning process. There are people who practice L2 so much that they become fluent, and the language becomes their first tongue. But for most people, it is quite difficult to get or learn any other lingo. It might take a whole life for some people to learn a second tongue.
Children master their mother tongue by the age of 5, except for a few grammatical structures. If they start learning a second tongue by the age of 7, they can become fluent. The learning speed of children is also considerably faster than adults.