How Does IRS Calculate Top Percent Income Earners?

When people say, top 1 percent of income earners, or top 5 percent, or top 10 percent of income earners, like Paul Craig Robert's article:

" In 2006, the most recent year for which data is available, Americans whose tax returns placed them in the top 1 percent earned 22.1 percent of adjusted gross income and paid 39.9 percent of all federal individual income taxes.
The top 5 percent, defined as rich by President Obama, paid 60.1 percent of all federal individual income taxes. The top 10 percent paid 71 percent.
Those Americans whose earnings placed them in the bottom half of the income distribution paid less than 3 percent of the individual income tax collected. "

http://www.taxfoundation.org/news/show/250.html

Is it top percent out of the entire American population? In other words, are they including children and adults who don't work?