According to ancient documents, Tang Yuan is not more than 1500 years old in China. There was no any recording about Tang Yuan in the documents before Song dynasty (960 - 1279). In Han dynasty (202 B.C - 220) Chinese ate rice gruel and sweet cakes on Yuan Xiao festival. There was not any written clue about Tang Yuan from the documents in South and North dynasty (420 - 589) and Sui dynasty (581 - 618). In Tang dynasty, however, there was a kind of food similiar with Tang Yuan. The food is probably the precessor of Tang Yuan. It was made of date flour and filling, and was fried instead of cooked.
Recordings from Song dynasty shows that Chinese at that time had over 20 kinds of snacks for Yuan Xiao celebration. Tang Yuan was one of them. The name, the producing method and the meaning of Tang Yuan in Song dynasty were just as same as today. With other words, Chinese have kept the same Chinese New Year custom for about 1000 years.
However, today Tang Yuan are no longer only Chinese New Year food. They have become daily food which you can find in supermarket in China anytime. In addition, there are more and more Tang Yuan varities, because there are more and more new food materials in China. Tang Yuan are delicious and beautiful, and it is easy to cook them. They have become a popular fast food in China.