There is nothing in the tax code that ties an adult child's limit being tied to the limit of the parents. The tax code only addresses the limit as it applies to spouses, splitting the family limit between the spouses. The result is that the adult child is covered by a family HDHP. The adult child is also not the spouse of either of the parents, so the adult child is not subject any limits that apply to spouses. This result came about when the ACA changed the law to allow unmarried nondependent children to remain on their parents' health insurance plan until age 26 (and did not change section 223 to make a single family limit apply to all of those covered under a particular HDHP, even if not a spouse).
