As long as you are a US Citizen the IRS expects you to report your world wide income and pay tax accordingly. If you are living on Social Security benefits and have no other income you will owe no income tax, Social Security benefits gradually become taxable up to 85% of the total benefits in the higher income tax brackets. Suppose a couple, age 65, retires on $2000/month SS and has old 401K or other retirement savings worth $100,000. In 2013 they could take distributions from the retirement accounts of $22,400 and still not owe any income taxes. And if they do not need any more money, they could rollover the amounts into ROTH IRA's. This has the advantage of growing tax free, so that any time after 5 years of establishing the account you can take out as much as you like from the ROTH IRA and pay no taxes. Because of the 5 year rule, it is best to start a ROTH IRA while still having earned income before you retire to get the clock ticking. For a single person aged 65 $11,500 in adjusted gross income means no taxes, $10,000 if under age 65. For a couple if both are under age 65 the amount is $20,000.
The above info is a simplified example of how to get the most out of paying no taxes, and all income must be considered, including the $3.00 an hour you earn teaching English, the huge profits you make selling items on Amazon, investment income etc.
When you turn 70.5 you must start taking Required Minimum Distributions from your traditional IRA's and 401k's, but not from ROTH IRA's
Hope this gets you started in thinking how to best plan and fund your golden years.
Disclaimer: I offer this info purely to help those of you not well versed in these matters, I am not looking to get new tax clients nor am I endorsing any particular investment products, just telling you what the tax code says today.