I have a tincture batch of castor/glycerin (approximately 40/60 by volume) that is 6 years old. After taking off the tincture, can the residual castor be used in formulation the same as full strength? Or is it lesser than full strength now?

Everything used as the mother base will have some strength loss after tincturing it. It will still have odor just weakened a bit. It depends upon the initial amount and the type of active used to the tincture ratio that determines the residual strength that remains. Some materials can be tinctured more than once and still have a good product odor representation.