Mitt Romney's gotten the "fact" that 47 percent of people don't pay taxes back in the news, so lets look at this Tax Policy Center breakdown for more information on the subject.
As we can see, it is true that 47 percent of the population pays no net federal income tax. Many of those people do pay federal payroll taxes on their income, however. Of those who pay neither income nor payroll taxes, most are elderly. That's because elderly people generally don't have jobs. Make of this what you will, but in terms of partisan politics it seems very likely that a large share of these elderly freeloaders are actually Romney voters. Presumably in their own heads they don't think of themselves as freeloaders because they paid taxes in the past. But that's generally going to be the case. At one point in college I wasn't paying any taxes, but then I graduated and my earnings went up so now I pay plenty of taxes. It's the circle of life.