If your income is so low that you aren’t required to file a tax
return, then you're automatically exempt from the penalty. For example,
if a single taxpayer’s income in 2016 was less than $10,350, there
typically was no need to file a return; for married couples, the cutoff
was $20,600. Even if you do file a return -- to claim a refund, for
example, or because you are self-employed and earned enough money to
require it -- you're still exempt from the insurance requirement if your
income is below the cutoff.
You're also exempt from the requirement if the most inexpensive
coverage you can find would cost you more than 8.13% of your household
income.
From: https://turbotax.intuit.com/tax-tools/tax-tips/Health-Care/Are-You-Exempt-From-Health-Care-Coverage-...
