Vitamin D deficiency is more common than you might expect. People who don't get enough sun, especially people living in Canada and the northern half of the US, are especially at risk. However, even people living in sunny climates might be at risk, possibly because people are staying indoors more, covering up when outside, or using sunscreens to reduce skin cancer risk.
Older people are also at risk for vitamin D deficiency. They are less likely to spend time in the sun, have fewer "receptors" in their skin that convert sunlight to vitamin D, may not get vitamin D in their diet, may have trouble absorbing vitamin D even if they do get it in their diet, and may have more trouble converting dietary vitamin D to a useful form due to kidney problems. In fact, some scientists suggest that the risk for vitamin D deficiency in people over 65 years of age is very high. As many as 40% of older people living in sunny climates such as South Florida might not have optimal amounts of vitamin D in their systems.
Vitamin D supplements may be necessary for older people, people living in northern latitudes, and for dark-skinned people who need extra time in the sun, but don't get it. Talk to your health care provider about whether a supplement is best for you.