During old age, the decline of various organ systems becomes manifest.

Ageing occurs at different speeds in different individuals dependant on many factors including genes, lifestyle and chronic illness. 

The physical manifestations of ageing include:

weight loss,

physical fatigue,

muscle loss and

slowing of cognitive function,

internally all the organs of the body are also declining in function. 

Previous health issues and lifestyle choices produce a different constellation of problems and symptoms in different people. The decline in the reserve in internal organs makes the elderly more prone to develop  complications from mild problems (such as dehydration from a mild gastroenteritis). Multiple problems may compound: a mild fever in an elderly person may cause confusion, which may lead to a fall and a fracture of the hip.

In older people it is important to distinguish between the effects of normal ageing from the development of an illness. For example, some loss of kidney function is a normal part of ageing, but kidney failure is not.
 
Older people often take multiple medications. In cases of multiple medical conditions, people may also be taking self-prescribed herbal medications and/or over-the-counter drugs. Polypharmacy (taking of multiple medications which is common in the older person) may increase the risk of drug interactions or unwanted bad effects.