In theory at any age you wish.
However, exposure to the constant noise is not a good idea, so they do need to be able to wear some form of ear defenders. As with all children some will suffer from travel sickness and some may not.
Theory aside, I would also want to consider the outcome of an engine failure and forced landing as unlikely as this may be or any other reason that might arise to make an emergency evak. Getting a child that is strapped into the aircraft out of the back seat may not be the most straight forward of operations.