Social Security is an earned benefit. You become eligible by paying the payroll tax during your working years, and the amount you get is geared to your wage history - with a glaring exception.

Social Security has a big birthday coming up next year -- it's turning 90! It's kept many millions of people out of poverty for decades and was recently supporting more than 71 million people with around $1.5 trillion in benefits for the year.

In 1983, Congress passed a law that gradually increased the age at which people may receive 100% of their Social Security benefits from 65 to 67, a change that was felt over the following decades as Americans approached their retirement age, according to Bipartisan Policy Cente r. The increase aimed to match increasing life expectancy.

The age at which full retirement benefits can be received rises next year, as experts warn those preparing to retire not to be caught out.