You need more vitamin D as you get older. The Institute of Medicine recommends that people aged 51 to 70 should get 600 IUs of vitamin D each day. People over 70 should have 800 IUs per day.
Image courtesy of The 2004 Surgeon General's Report on Bone Health and Osteoporosis: What It Means to You. U.S. Department of Health and Human Services, Office of the Surgeon General, 2004.