At what age should a child begin to be taught to pray?

Seven years, according to the wording of the hadith.
The Prophet ﷺ said: command your children to pray when they are seven years old, and strike them over it when they are ten, and separate their sleeping places. A narration in Sunan at-Tirmidhi gives the same wording: teach the child prayer when he is seven years old. Notice what is mentioned alongside that command — separating sleeping places — so the upbringing in prayer comes together with the upbringing in boundaries. See Educating the family.