Some random other country doesn't owe you a thing and so they get to be selective about who they let in. In Canada you have to show you won't place "excessive demand" on their services, and it's defined as:

"Requirements for medical or social services of such a level as to threaten the sustainability of Canada’s publicly funded healthcare system, or deny or delay access to Canadians that would prevent the admission of an applicant to Canada."

http://www.cic.gc.ca/english/resources/publications/dmp-hand...

Some random web link says that Excessive Demand was calculated to be $24,030 of health or social services over a five-year period. There's lots of stories of families with a mildly autistic child being denied residence in Canada, for example.

http://www.canadianimmigrant.ca/health/healthinsurancecovera...

It's not in the best interests of a country to let an older person in who wont pay much tax and then try to collect social security and use health services as they get older.

Australia, for example, has a specific visa to enable retirees to live in Australia for a period of time. Note, however, that you need a lot of assets, a substantial income and proof of health insurance for the duration of the stay.

http://www.immi.gov.au/visitors/retirement/405/