I have a 25 year-old dependent on my health plan. Can I add them to the HSA/HDHP?

In 2010, Health Care Reform Law extended health plan eligibility to children of the covered employee until the child turns 26 years of age, so you can cover your 25 year-old on your HDHP medical plan.  However, the HSA is subject to IRS rules and guidelines, and since their definition of dependent is “up to age 24 if a full-time student,” you are not able to use your HSA funds to pay for out-of-pocket medical expenses for that dependent, even though they may be covered on your HDHP for medical expenses.