Most older adults who have financial and social resources can avoid long term residential care for quite a while, while those without family or spouse support and with less ability to pay for in-home help may land in a nursing home sooner. Cognitive abilities factor largely - someone with physical frailty who is clear of mind can stay at home with supports much longer than someone who loses the ability to remember basic things and loses normal judgement about dangers, etc. Thus, various dementias are a huge reason that people need to be admitted to care facilities. Reasons for admission other than cognitive ones include falls and breaking bones, especially hips, but other bones as well, that suddenly render the person dependent on others for ADLs, or stroke, which likewise debilitates many older adults. And families which happily care for older people in their own home or the family's home can be challenged by incontinence in combination with frailty or with cognitive impairment. That often figures into a family's need to place someone. Finally, at the end of life, people at very advanced ages who have thus far been in fairly good shape cognitively and physically can become so frail and rather ill from old age (in the mid-90s or beyond) that a nursing home setting becomes a comfort to the person and any family for caring for that person's needs in the final months or weeks. Certainly, there will be research reporting particulars about these factors that can be searched. Good luck!