* Mandarin is a dialect, not a language. While often referred to as "the Chinese language," Mandarin is actually one of many dialects spoken in China.
* Dialects have evolved over time. The languages and dialects spoken in China have been evolving for millennia. It's not possible to pinpoint a specific date when Mandarin "began."
* Mandarin became dominant in the 20th century. The modern Standard Mandarin, based on the Beijing dialect, rose to prominence in the 20th century due to the political unification of China and its promotion as the official language.
So, rather than a "start" date, it's more accurate to say that:
* Mandarin has a long history, with roots dating back to the Middle Ages.
* The modern Standard Mandarin emerged in the 20th century.
It's important to remember that many other Chinese dialects continue to be spoken by millions of people across China.