As children we all had to study for and take a "vocabulary tests" - learning a new list of words, their spelling and definitions, every week. As we continued on in school, read more books and studied more subjects, our vocabulary naturally expanded.
The great thing and the hard thing about a truly "living language"; like English, is that it is always changing, adopting, adapting, and adding new words, new concepts, new elements.
How many words do you use in everyday discussions in 2014 that a few years ago would have had no meaning whatsoever? When did you first learn to speak "coffee"; so that you could communicate your beverage choice with the barista. Phrases such as "venti, black-eye, half-cap, mocha frappuchino"; that would have been complete gibberish a couple decades ago now come tripping off the tongue without need for an interpreter.