I began to learn English when I was third grade in the primary school, it is common that children study forigen language when they were eight years old, but nowadays, more and more children learn it at earlier age, even as soon as they can speak words.
however, i definetly agree with that begin a foreigen language at early age. For one thing, the earlier children learn it , the easier the get it. we all know that it is the crucial moment of person's childhood, they can  accept something new easily and they are full of passion for the new knowledge, for instance, my young brother, learn English when he was only 5 years old, now, he speak English as fluent as he can and communicate with foreigner without any problem. For another, if children learn foreign language early, it si good for our country expand global education and excite children broaden their eyesight, they can contact with foreigen culture easily. however, this kind of ethusaism to learn foreign language should not be so crazy that neglect our mother tongue and native culture,one of my friend who learn English when he start to speak word, the first word he said is not the "mum" in Chinese, but "mother" in English, his parents taught him foreign language, now even sometimes ,he cannot understand the meanign of a Chinese word but learn well in English. So it is essential that not only know our mother tongue but also gasp and exert it in every field expertly, that is the basic for begining a foreigen language and new culture.
in my view piont, it has many advantagies for children to learn foreigen language at their early age, and on the other hand, the mother tongue should be put in the first place, then the best time when child begin learning foreign language is that they absolutely know their mother tongue and have the ability to understand new knowledge.