Language quite possibly evolves more then just about anything else that humans do on a day-to-day base. Most language evolves from new generations coming up with new words, sayings, or phrases that catch on and become commonly used.
For example, even just a hundred or so years ago the language is vastly different from what is heard today, which is all the proof needed to show that language evolves just as any one species does.
I don’t believe that the language of the individual changes very much, but instead more so the generation gaps differ the most, the way that a group of third graders may talk would be different from the way anyone in high school would speak to each other, which would be completely off from how anyone over the age of sixty spoke. The language evolves slowly, although hints of it may be shown by generation gaps, it is more than visible from century to century.