Affordable Care Act
When the Affordable Care Act was enacted in 2010, it included a provision that children be allowed to remain on their parent's health insurance plan until age 26.
This created a unique HSA provision that allowed those individuals to open an HSA and contribute as long as they were no longer tax-dependent (not claimed by anyone as a dependent on their taxes). This is an opportunity for young people to create a long-term health savings vehicle that can be used or invested for many years.
HSA eligibility requirements
Individuals who are covered by their parent's health plan
The parent's plan must be an HSA-eligible, high-deductible health plan (HDHP)
Less than 26 years of age
Cannot be claimed as dependent on anyone else's taxes
Has no other health insurance (other than parent's health plan)
Is not enrolled in Medicare
Not required for HSA-eligibility
It's not required that the individual's parents have an existing HSA or contribute to their HSA.
Parents can contribute to their HSA at any level.
Family maximum limit
Eligible individuals in this situation can contribute to the current year's family contributions limit, assuming that the parent's health plan covers the parent and the child. This situation is unique in that:
Individuals can open their own HSA, separate from their parent's.
But are allowed to contribute to the family maximum.
Note on parent HSA coverage
Parents can use their HSA funds to cover the qualified medical expenses of children claimed as dependents; parents can not use their HSA funds to cover the expenses of children not claimed as dependents on the parent's tax return. (Some narrow exceptions apply; check them out here.)
In other words:
A non-dependent child can open up a separate HSA.
However, a parent cannot use their HSA funds to cover the expenses of a non-dependent child.
Recap
Adult children can remain on their parent's health plan until age 26.
Non-dependent children under 26 can open their own separate HSA if they stay on a parent's HSA-eligible plan. In this situation, they can contribute up to the family maximum contribution limit.
A parent's HSA can not cover the expenses of a non-dependent child.