Discount brokerages require a minimum age of 18. However, you can create a sub-account under your dads name (as long as he signs some sort of consent form) and these sub-accounts don't have a minimum age requirement.
Vitamin D deficiency rickets can be caused by a lack of vitamin D in the diet, a lack of exposure to the sun, or malabsorption syndromes in which there is an inability of the intestines to adequately absorb nutrients from foods.
Low Vitamin D levels are caused by two very different reasons. Low vitamin D is common in people without any other health problems, but low vitamin D can also be caused by a parathyroid tumor. The connection between parathyroid disease and low Vitamin D can be confusing, even for many doctors. Parathyroid tumors cause low Vitamin D, not the other way around.