Hi,
Itchy skin below knees along with generalized itching all over the body could be due to dry skin, thyroid disorders, liver disease, kidney disease or anemia. Certain medications can also cause itching. I would recommend that you consult a physician for further work up and confirmation of the cause for this. Best.
My Aunt had the same problem and the doc told her that when she gets out of the bath tub or shower, don't dry off and immediately apply lotion and just air dry. It doesn't have to be expensive lotion, just any kind will do. She does that and it has helped her tremendously. Good luck!!