I have visible blue veins on the back of my knees and some on my upper thighs. I want to know if that is normal? I am 16 (Photo)

It is not normal but with ever increasing acquired risk factors such as use of BCP, obesity and lack of regular leg exercises (walking) and also genetics (hereditary risk), it is becoming more prevalent in younger patients. It does not appear that you have a weight problem from the pictures. BCP are used for acne control and regulating periods in women of chidbearing age. Read my book about venous insufficiency on the link below. Dr Karamanoukian

Increasing leg veins can occur at any age and when it occurs in the under 20 year olds, usually the most common cause is genetics.  I would recommend that you see a vein specialist for a baseline venous evaluation.  From the pictures it seems that the best treatment would be either support hose (which I doubt you will wear at age 16) or localized treatment with sclerotherapy.