The Bible gives no age for when a child is ready to be baptized. For baptism to be a meaningful experience and expression of faith, its significance should be understood. Our desire is for a child to understand this important step of baptism in his/her life.
In child development, typically when children reach the age of 8 or 9, they gain the ability to understand abstract concepts. It is at this stage in a child development that they gain the ability to understand imagery such as those concepts involved in baptism representing our spiritual death, burial and resurrection. That being said, each baptism should be determined on a child-by-child basis.