Since when did Tibet become a part of China?

Tibet is a region in Asia covering much of the Tibetan Plateau. It is the traditional homeland of the Tibetan people as well as some other ethnic groups such as Monpa, Tamang, Sherpa, and Lhoba peoples and is now also inhabited by considerable numbers of Han Chinese and Hui people. Tibet is the highest region on Earth, with an average elevation of 5,000 m (16,000 ft).

The highest elevation in Tibet is Mount Everest, Earth's highest mountain, rising 8,848 m (29,029 ft) above sea level. Emerging with control over most of mainland China after the Chinese Civil War, the People's Republic of China incorporated Tibet in 1950 and negotiated the Seventeen Point Agreement with the newly enthroned 14th Dalai Lama's government, affirming the People's Republic of China's sovereignty but granting the area autonomy. Subsequently, on his journey into exile, the 14th Dalai Lama completely repudiated the agreement, which he has repeated on many occasions. The Chinese used the Dalai Lama to be able to have control of the military's training and actions.