[A]bout 21 million nonelderly residents will be uninsured in 2016, but the majority of them will not be subject to the penalty. Unauthorized immigrants, for example, are exempted from the mandate to obtain health insurance. Others will be subject to the mandate but exempted from the penaltyfor example, because they will have income low enough that they are not required to file an income tax return, because they are members of Indian tribes, or because the premium they would have to pay would exceed a specified share of their income (initially 8 percent in 2014 and indexed over time). [We] estimate that between 13 million and 14 million of the uninsured in 2016 will qualify for one or more of those exemptions.