At what age is it appropriate to begin to teach a child/children the Gnostic studies and accompanied spiritual exercises? Are there Spiritual exercises that are not suitable for children?
It depends upon the child. Each one has their own needs and capabilities.
Children learn meditation very easily, if they are taught properly.
One should be mindful of their age. The sexual mysteries can be explained when they are of appropriate age. Everything else in Gnosis can be studied at any age.