The IRS extended the ability of a parent to use FSA and HRA funds for a child who has not reached age 27 by the end of the taxable year, but this guidance did not extend to HSAs. Congress or the IRS may provide additional guidance on this issue in the future. (10 weird HSA FAQs – Self-Funded Insurance Experts | Diversified Group Blog)

"The IRS extended the ability of a parent to use FSA and HRA funds for a child who has not reached age 27 by the end of the taxable year, but this guidance did not extend to HSAs. Congress or the IRS may provide additional guidance on this issue in the future."

https://info.dgb-online.com/2017/02/07/10-weird-hsa-faqs/