Also, although the ACA requires people to maintain health insurance or else pay a penalty, the law allows a series of exemptions and special considerations, meaning that millions of individuals could be exempt from the requirement to maintain coverage.
The mandate’s exemptions cover a variety of people, including: members of certain religious groups and Native American tribes; undocumented immigrants (who are not eligible for health insurance subsidies under the law); incarcerated individuals; people whose incomes are so low they don’t have to file taxes (currently $9,500 for individuals and $19,000 for married couples); and people for whom health insurance is considered unaffordable (where insurance premiums after employer contributions and federal subsidies exceed 8% of family income).
http://healthreform.kff.org/notes-on-health-insurance-and-reform/2012/march/the-individual-mandate-how-sweeping.aspx