The precursors for vitamin D are reduced as we age. By age 70, our ability to produce vitamin D is about half of what it was at age 20. D is in scarce supply in our regular diets. Most milk and some juices, milk alternatives and cereals are fortified with D, but other dietary sources — fatty fish like mackerel and sardines, and some mushrooms — aren’t exactly a staple in most American diets. As a result, nearly 1 in 4 people in the U.S. have inadequate blood levels of vitamin D3, the most active form.

How Is Your Vitamin D Status – link above

Yikes.