When it comes to the world's retirement systems, each is not necessarily created equal.

Pension systems around the world, including social security systems and private sector arrangements, are now under more pressure than ever as more people enter retirement, according to a new report. While some systems are structured to withstand the pressure, others cannot.

"The major causes of this demographic shift are declining birth rates and increasing longevity," stated the report, which was issued by Mercer, the global consulting firm, in partnership with the Australian Centre for Financial Studies.

The 2018 Melbourne Mercer Global Pension Index compared 34 retirement income systems worldwide, measured against 40 indicators.

One nation that did not finish in the top 10 of the ranking was the U.S., which placed 19th with a grade of "C," which the report defined as "a system that has some good features, but also has major risks and/or shortcomings that should be addressed."

Each nation's pension system was graded on "adequacy," including benefits provided; "sustainability," which includes the system's total assets and the percentage of people covered; and "integrity," which includes how well the systems are regulated and administered. The U.S., for example, scored C grades in all three categories.

The following nations, in ascending order, were ranked by the report as having the world's best retirement systems:

10. Canada

Canada’s retirement income system is based on a universal flat-rate pension, supported by a means-tested income supplement. Canada earned an overall grade of B, including a B+ for system integrity and a B for adequacy.