Dependents under tax laws are not restricted to children. Theoretically, one adult can claim another adult as a dependent under the federal laws. So that factor has nothing to do with whether you or your parents bear responsibility for a medical bill (which would . If you were 18 when the bill was incurred, you may be held responsible. The only wrinkle to this could be if you were covered under your parents' medical insurance. However, my best guestimate is that you would still remain responsible for whatever wasn't covered by the insurer.