NEW YORK (MainStreet) — "Boomerang children," young adults who move back in with their parents, often due to financial reasons, may unwittingly expose their parents to tax penalties, if the youngsters don't get health insurance qualified under the Affordable Care Act (ACA) come January.

Nearly 30% (3 in 10) of young adults ages 25 to 34 have moved back home, according to a 2012 Pew Research Center survey.

Under the individual shared responsibility provision of the ACA starting January 1, 2014, Americans are required to have minimum essential health coverage each month, unless they are qualified for an exemption. A parent of an adult child living at home who can be claimed as a dependent is liable for a non-exempt, uninsured dependent, even if the parent doesn't claim the child as dependent.

In states with expanded Medicaid, the child becomes part of the household and therefore will not qualify for Medicaid coverage, if the household income is above certain thresholds.

"This subset of 'young invincibles' may well cost their parents more than a spare bedroom," says Brian Haile, senior vice president of health care policy at Jackson Hewitt Tax Service.

The penalty, in general, is the total monthly penalties or the total monthly cost of the national average Bronze plan premiums for the family, whichever is less. For a couple in New Jersey with an income of $70,000, that would amount to $458 in federal penalties, according to a Jackson Hewitt Tax Service report co-authored by Haile.

There are exceptions, including hardship (which must be certified by the health insurance Marketplace) and in cases where coverage is unaffordable because the cost of coverage is more than 8% of household income. In states where there is no Medicaid expansion, these adult children may fall through the cracks and not qualify for tax credits intended to make Marketplace health insurance coverage affordable. This "gap group," however, will not be responsible for paying the penalty for not having health insurance coverage, according to the Centers for Medicare & Medicaid.

--Written by S.Z. Berg for MainStreet