What is the age when children should start fasting?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

As agreed upon by the Muslim scholars, it is not obligatory for young children to do any physical act of worship until they reach the age of puberty.  The Prophet, sallallaahu ‘alayhi wa sallam, said concerning this: “The pen is lifted for three (categories): one who is sleeping until he wakes up, a child until he reaches the age of puberty, and one who has lost his mind until he comes back to his senses.” [Abu Daawood and At-Tirmithi: good]

Nevertheless, children should be ordered to observe the prayers and fast from the age of 7 so that they can get used to it. They should be beaten if they do not do so by the age of 10. A child is not to be (very lightly) beaten for not fasting unless he is capable of bearing it (i.e. fasting).

Allaah Knows best.