An alternative view of the topic:

These kinds of studies are based on a premise that I believe is somewhat faulty and ageist in nature. There is a difference between functional ability, perceived health, and pain or discomfort related to illness/disease and getting old. To equate these with age or that of feeling healthy with youth is a faulty presupposition. Yet, many still persist in holding this as an acceptable truth. Too often these subtle beliefs manifest themselves in negative attitudes toward aging. I try to live by the thought that I am not who I was yesterday or who I will be tomorrow. On any given day I may have more or less of a given ability. I may feel more or less pain and discomfort. Age is not the way I choose to define this phenomenon.