Vitamin D and the impact on bone health of seniors
Senior citizens are prone to having low levels of vitamin D. This is a serious issue which could lead to devastating consequences such as falls, fractures and osteoporosis.
Vitamin D and falls in older individuals
An analysis of several double-blind randomized clinical trials demonstrated that by supplementing with 700 to 1000 IU vitamin D per day, an older people reduce their risk of falling by 20 per cent, whether or not they were also taking calcium supplements 
Vitamin D and fractures in older individuals
Vitamin D in combination with calcium is often used in an attempt to decrease the risk of fractures in bones other than the vertebrae in older individuals  particularly those in institutional care. An analysis of multiple vitamin D studies determined that a dose of vitamin D 400 IU per day or greater is helpful for individuals aged 65 years or older.
Vitamin D and osteoporosis in older individuals
Osteoporosis consists of a progressive loss of bone mass. Most of our bone mass is built up from the age of 10 to 20 or 30, and after that, we start losing bone mass.  If too much bone mass is lost as we get older, our bones become fragile and may fracture more easily. Osteoporosis is a devastating and costly disease affecting 10 million Americans; another 34 million have low bone mass, making them more susceptible to osteoporosis. Without intervention, one in two women and one in three men age 50 and above will experience a fracture due to osteoporosis. The hip, spine, and wrist are the most common sites of fracture, with the hip fracture having the most devastating complications.
Surprisingly, only 20 per cent of people who have suffered a hip or other fracture due to osteoporosis receive treatment. 
We are not able to provide you with a recommendation for how much vitamin D one should take on a daily basis. We recommend that you speak with a healthcare professional about your personal situation. In the meantime, here is a brief summary of the daily vitamin D supplementation guidelines by various medical organizations:
Osteoporosis Canada also suggests that vitamin D supplementation should take place all year round.
 Dawson-Hughes, A. Mithal, J.-P. Bonjour, S. Boonen, et al., Osteoporosis International, IOF position statement: vitamin D recommendations for older adults, July 2010, Volume 21, Issue 7, pp 1151-1154, First online: 27 April 2010
 Avenell A1,Mak JC, O’Connell D. Vitamin D and vitamin D analogues for preventing fractures in post-menopausal women and older men. Cochrane Database Syst Rev. 2014 Apr 14;4:CD000227. doi: 10.1002/14651858.CD000227.pub4.
 Tom R. Hill, Terence J. Aspray and Roger M. Francis (2013). Vitamin D and bone health outcomes in older age. Proceedings of the Nutrition Society, 72, pp 372-380. doi:10.1017/S0029665113002036.
 Heike A. Bischoff-Ferrari, DrPH, Walter C. Willett, DrPH, John B. Wong, MD et al. Arch Intern Med. 2009;169(6):551-561. doi:10.1001/archinternmed.2008.600. Prevention of Nonvertebral Fractures With Oral Vitamin D and Dose DependencyA Meta-analysis of Randomized Controlled Trials. March 23, 2009. https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/414883
 Hightower L., Osteoporosis: pediatric disease with geriatric consequences, Orthop Nurs.2000 Sep-Oct;19(5):59-62.
 NIH Osteoporosis and Related Bone Diseases National Resource Center, Kids and Their Bones, March 2015, https://www.bones.nih.gov/health-info/bone/bone-health/juvenile
 RichardDell, Denise Greene, Steven R. Schelkun et al. Osteoporosis Disease Management: The Role of the Orthopaedic Surgeon. J Bone Joint Surg Am, 2008 Nov; 90 (Supplement 4): 188 -194 . https://insights.ovid.com/crossref?an=00004623-200811004-00018
 Nicholas Harvey, Elaine Dennison & Cyrus Cooper , Osteoporosis: impact on health and economics, Nature Reviews Rheumatology6, 99-105 (February 2010) |doi:10.1038/nrrheum.2009.260
 The Institute of Medicine, DRIs for Calcium and Vitamin D. November 30 2010.
 Health Canada. Vitamin D and Calcium: Updated Dietary Reference Intakes. March 22, 2012. https://www.canada.ca/en/health-canada/services/food-nutrition/healthy-eating/vitamins-minerals/vitamin-calcium-updated-dietary-reference-intakes-nutrition.html#a9
 National Osteoporosis Foundation, Calcium and Vitamin D: What You Need to Know. https://www.nof.org/patients/treatment/calciumvitamin-d/#howmuchvitamind
 Osteoporosis Canada, Healthy Eating for Healthy Bones